xfs_healer: use getmntent to find moved filesystems

xfs_healer: use getmntent to find moved filesystems

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.  When this happens,
weakhandle::mountpoint becomes obsolete and opening it will either fail
with ENOENT or the handle revalidation will return ESTALE.

However, we do still have a means to find the mounted filesystem -- the
fsname parameter (aka the path to the data device at mount time).  This
is record in /proc/mounts, which means that we can iterate getmntent to
see if we can find the mount elsewhere.

As documented a few patches ago, this would be easier if we had
revocable fds that didn't pin mounts, but that's a very huge ask.

This getmntent code enables xfs_healer to find a filesystem that has
been bind mounted in a new place and the original mountpoint detached:

# mount /dev/sda /mnt
# xfs_healer /mnt &
# mount /mnt /opt --bind
# umount /mnt

The key here is that each bind mount gets a separate struct mount
object.

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