xfs_scrub: create infrastructure to read verify data blocks

xfs_scrub: create infrastructure to read verify data blocks

Manage the scheduling, issuance, and reporting of data block
verification reads.  This enables us to combine adjacent (or nearly
adjacent) read requests, and to take advantage of high-IOPS devices by
issuing IO from multiple threads.

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>