dm: don't start current request if it would've merged with the previous

Request-based DM's dm_request_fn() is so fast to pull requests off the
queue that steps need to be taken to promote merging by avoiding request
processing if it makes sense.

If the current request would've merged with previous request let the
current request stay on the queue longer.

Suggested-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>