verify: always log IO in the order they are issued

verify: always log IO in the order they are issued

We currently log verify_backlog verifies when they complete,
which means the sequence of verify and issue might be different.

Change this to log in one spot, prior to issue, and track the
completion state of the logged unit instead. This unifies the
handling of verifies.

Signed-off-by: Jens Axboe <axboe@fb.com>