btrfs: test xattr changes for RO btrfs property

btrfs: test xattr changes for RO btrfs property

Test creation, modification and deletion of xattr for a BTRFS filesystem
which has the read-only property set to true.

Re-test the same after read-only property is set to false.

This tests the bug for "security.*" modifications which escape
xattr_permission(), because security parameters are usually let through
in xattr_permission, without checking
inode_permission()->btrfs_permission().

This resulted in being able to change a security xattr on a RO subvolume
until it got fixed by kernel commit b51111271b03 ("btrfs: check if root
is readonly while setting security xattr").

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>