Fix batch mismatch for verifies

Fix batch mismatch for verifies

If we set verify_backlog=32, we expect 32 writes followed by 32
reads that verify the written data. But if we fail getting a
verify io_u, then we may decrement the batch counter when we should
not. The result is that we write 32, read 32, write 1, read 1, repeat.

Signed-off-by: Jens Axboe <axboe@kernel.dk>