xfs_info: limit findmnt to find mounted xfs filesystems

xfs_info: limit findmnt to find mounted xfs filesystems

When running xfstests with -overlay, the xfs mount point
(a.k.a $OVL_BASE_SCRATCH_MNT) is used as the $SCRATCH_DEV argument
to the overlay mount, like this:

/dev/vdf /vdf xfs rw,relatime,attr2,inode64,noquota 0 0
/vdf /vdf/ovl-mnt overlay rw,lowerdir=/vdf/lower,upperdir=/vdf/upper...

Ever since commit bbb43745, when xfs_info started using findmnt,
when calling the helper `_supports_filetype /vdf` it returns false,
and reports: "/vdf/ovl-mnt: Not on a mounted XFS filesystem".

Fix this ambiguity by preferring to query a mounted XFS filesystem,
if one can be found.

Fixes: bbb43745 ("xfs_info: use findmnt to handle mounted block devices")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>