I've got a new i/o scheduler in testing, some changes where needed in
the block layer to accomodate it. Basically because right now
assumptions are made about q->queue_head being the sort list. The
changes in detail:

o elevator_merge_requests_fn takes queue argument as well

o __make_request() inits insert_here to NULL instead of
  q->queue_head.prev, which means that the i/o schedulers must
  explicitly check for this condition now.

o incorporate elv_queue_empty(), it was just a place holder before

o add elv_get_sort_head(). it returns the sort head of the elevator for
  a given request. attempt_{back,front}_merge uses it to determine
  whether a request is valid or not. Maybe attempt_{back,front}_merge
  should just be killed, I doubt they have much relevance with the wake
  up batching.

o call the merge_cleanup functions of the elevator _after_ the merge has
  been done, not before. This way the elevator functions get the new
  state of the request, which is the most interesting.

o Kill extra nr_sectors check in ll_merge_requests_fn()

o bi->bi_bdev is always set in __make_request(), so kill check.