btrfs: introduce btrfs specific idmapped mounts tests

btrfs: introduce btrfs specific idmapped mounts tests

While core vfs functionality that btrfs implements is completely covered
by the generic test-suite the btrfs specific ioctls are not.
This patch expands the test-suite to cover btrfs specific ioctls that
required changes to work on idmapped mounts. We deliberately don't use
the libbtrfsutil library as we need to know exactly what ioctl's are
issued and we need to be in control of all privileges at all times. This
test-suite currently tests:

- BTRFS_IOC_{SNAP,SUBVOL}_CREATE_V2
  - subvolume creation on idmapped mounts where the fsids do have a
    mapping in the superblock
  - snapshot creation on idmapped mounts where the fsids do have a
    mapping in the superblock
  - subvolume creation on idmapped mounts where the fsids do not have a
    mapping in the superblock
  - snapshot creation on idmapped mounts where the fsids do not have
    a mapping in the superblock
  - subvolume creation on idmapped mounts where the caller is
    located in a user namespace and the fsids do have a mapping
    in the superblock
  - snapshot creation on idmapped mounts where the caller is located
    in a user namespace and the fsids do have a mapping in the
    superblock
  - subvolume creation on idmapped mounts where the caller is
    located in a user namespace and the fsids do not have a mapping
    in the superblock
  - snapshot creation on idmapped mounts where the caller is located
    in a user namespace and the fsids do not have a mapping in the
    superblock

- BTRFS_IOC_SNAP_DESTROY_V2
  - subvolume deletion on idmapped mounts where the fsids do have a
    mapping in the superblock
  - snapshot deletion on idmapped mounts where the fsids do have a
    mapping in the superblock
  - subvolume deletion on idmapped mounts where the fsids do not have a
    mapping in the superblock
  - snapshot deletion on idmapped mounts where the fsids do not have
    a mapping in the superblock
  - subvolume deletion on idmapped mounts where the caller is
    located in a user namespace and the fsids do have a mapping
    in the superblock
  - snapshot deletion on idmapped mounts where the caller is located
    in a user namespace and the fsids do have a mapping in the
    superblock
  - subvolume deletion on idmapped mounts where the caller is
    located in a user namespace and the fsids do not have a mapping
    in the superblock
  - snapshot deletion on idmapped mounts where the caller is located
    in a user namespace and the fsids do not have a mapping in the
    superblock
  - unprivileged subvolume deletion on idmapped mounts where the fsids
    do have a mapping in the superblock and the filesystem is mounted
    with "user_subvol_rm_allowed"
  - unprivileged snapshot deletion on idmapped mounts where the fsids do
    have a mapping in the superblock and the filesystem is mounted with
    "user_subvol_rm_allowed"
  - subvolume deletion on idmapped mounts where the caller is
    located in a user namespace and the fsids do have a mapping
    in the superblock and the filesystem is mounted with
    "user_subvol_rm_allowed"
  - snapshot deletion on idmapped mounts where the caller is located
    in a user namespace and the fsids do have a mapping in the
    superblock and the filesystem is mounted with
    "user_subvol_rm_allowed"

- BTRFS_IOC_SUBVOL_SETFLAGS
  - subvolume flags on idmapped mounts where the fsids do have a mapping
    in the superblock
  - snapshot flags on idmapped mounts where the fsids do have a mapping
    in the superblock
  - subvolume flags on idmapped mounts where the fsids do not have a
    mapping in the superblock
  - snapshot flags on idmapped mounts where the fsids do not have a
    mapping in the superblock
  - subvolume flags on idmapped mounts where the caller is located in a
    user namespace and the fsids do have a mapping in the superblock
  - snapshot flags on idmapped mounts where the caller is located in a
    user namespace and the fsids do have a mapping in the superblock
  - subvolume flags on idmapped mounts where the caller is located in a
    user namespace and the fsids do not have a mapping in the superblock
  - snapshot flags on idmapped mounts where the caller is located in a
    user namespace and the fsids do not have a mapping in the superblock

- BTRFS_IOC_INO_LOOKUP_USER
  - subvolume lookup on idmapped mounts where the fsids do have a mapping
    in the superblock
  - subvolume lookup on idmapped mounts where the fsids do not have a
    mapping in the superblock
  - subvolume lookup on idmapped mounts where the caller is located in a
    user namespace and the fsids do have a mapping in the superblock
  - subvolume lookup on idmapped mounts where the caller is located in a
    user namespace and the fsids do not have a mapping in the superblock

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Chris Mason <clm@fb.com>
Cc: David Sterba <dsterba@suse.com>
Cc: fstests@vger.kernel.org
Cc: linux-btrfs@vger.kernel.org
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>