Request insertion in the current tree is a mess. We have all sorts of
variants of *elv_add_request*, and it's not at all clear who does what
and with what locks (or not). This patch cleans it up to be:

o __elv_add_request(queue, request, at_end, plug)
      Core function, requires queue lock to be held

o elv_add_request(queue, request, at_end, plug)
      Like __elv_add_request(), but grabs queue lock

o __elv_add_request_pos(queue, request, position)
      Insert request at a given location, lock must be held