verify: ensure that verify_interval is always a factor of min/max bs

A previous commit force aligned the buffer length to the
verify_interval, but that breaks if max_bs isn't a multiple
of min_bs. If that isn't the case, we can align the block size
bigger than the IO unit really is. This causes corruption and
failures.

Fixes: a9f70b1f5087 ("Make sure io_u->buflen is aligned to the verify_interval")
Signed-off-by: Jens Axboe <axboe@fb.com>