xfs_scrub: dispatch the various phases of the scrub program

Create the dispatching routines that we'll use to call out to each
separate phase of the program.

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>