xfs_scrub: one read/verify pool per disk

xfs_scrub: one read/verify pool per disk

Simplify the read/verify pool code further by creating one pool per
disk.  This enables us to tailor the concurrency levels of each disk to
that specific disk so that if we have a mixed hdd/ssd environment we
don't flood the hdd with a lot of requests.

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>