xfs_growfs: ensure target path is an active xfs mountpoint

xfs_growfs: ensure target path is an active xfs mountpoint

xfs_growfs manpage clearly states that the target path must be
an active xfs mountpoint.

Current behavior allows xfs_growfs to proceed if the target path
resides anywhere on a mounted xfs filesystem. This could lead to
unexpected results. Unless the target path is an active xfs
mountpoint, reject it. Create a new fs table lookup function which
matches only active xfs mount points, not any file residing within
those mountpoints.

Signed-off-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>