xfs_healer: use statmount to find moved filesystems even faster

xfs_healer: use statmount to find moved filesystems even faster

As noted in the previous patch, it's possible that a mounted filesystem
can move mountpoints between the time of the initial mount (at which
point xfs_healer starts) and when it actually wants to start a repair.
The previous patch fixed that problem by using getmntent to walk
/proc/self/mounts to see if it finds a mount with the same "source"
name, aka data device.

However, this is really slow if there are a lot of filesystems because
we end up wading through a lot of irrelevant information.  However,
statmount() can help us here because as of Linux 7.0 we can open the
passed-in path at startup, call statmount() on it to retrieve the
mnt_id, and then call it again later with that same mnt_id to find the
mountpoint.  Luckily xfs_healthmon didn't get merged until 7.0 so it's
more or less guaranteed to be there if XFS_IOC_HEALTH_MONITOR succeeds.

Obviously if this doesn't work, we can fall back to the slow walk.

This statmount code enables xfs_healer to find a filesystem that has
had its mountpoint moved to a different place in the directory tree
without the use of bind mounts and without needing to walk the entire
mount list:

# mount -t tmpfs urk /mnt
# mount --make-rprivate /mnt
# mkdir -p /mnt/a /mnt/b
# mount /dev/sda /mnt/a
# mount --move /mnt/a /mnt/b

The key here is that the struct mount object is moved, and no new ones
are created.  Therefore, the original mnt_id is still usable.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>