block: Fix integrity verification

Commit bf36f9cf caused a regression by effectively reverting Nic's
fix from 5837c80e that ensures we traverse the full bio_vec list
upon completion.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jens Axboe <axboe@fb.com>