xfs_scrub: don't expose internal pool state

xfs_scrub: don't expose internal pool state

In xfs_scrub, the read/verify pool tries to coalesce the media
verification requests into a smaller number of large IOs.  There's no
need to force callers to keep track of this internal state, so just move
all that into read_verify.c.

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>