verify: don't adjust verification length based on interval when unaligned

It's perfectly fine to have a verification length that exceeds the
default verify interval when we have unaligned block sizes and
using block size ranges, since the verify interval just defaults to
the smallest blocksize.

Fixes: https://github.com/axboe/fio/issues/509
Signed-off-by: Jens Axboe <axboe@kernel.dk>