verify: use origina offset for verification

verify: use origina offset for verification

Requeued IO might have a different offset, since we increment it for
retry conditions. Store the original offset for verification purposes.

Just like we have io_u->xfer_buf/io_u->buf and
io_u->xfer_buflen/io_u->buflen, we really should be treating
io_u->offset the same. But that's a major change, so just add this
special original offset so we know that verify has it. Currently we
treat io_u->offset like we would have io_u->xfer_offset, so this just
makes io_u->verify_offset what io_u->offset should be.

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