xfs_info: use findmnt to handle mounted block devices

xfs_info: use findmnt to handle mounted block devices

Use findmnt to determine if the passed-in argument is associated with a
mount point, and if so, use spaceman to query the mounted filesystem.
If the user passed in a file, try to find out if it's a loop mounted
live filesystem and if so query the live filesystem.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>