xfs_scrub: refactor mountpoint finding code to use libfrog  path code

Use the libfrog path finding code to determine if the argument being
passed in is a mountpoint, remove all mention of taking a block device
(we have never supported that) from the documentation, and fix some
potential memory leaks.

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