This fixes a problem with the deadline io scheduler, if the correct
insertion point is at the front of the list.  This is something that we
never have gotten right in 2.4 either.

The problem is that the elevator merge function has to return a pointer
to a struct request, and for front insert we really have to return the
head of the list which cannot be expressed as a request of course.

The real issue is that the elevator_merge function actually performs two
functions - it scans for a merge, and if it can't find any, it selects
and insertion point.  It's done this way for efficiency reasons, even if
the design isn't all that clean.

So we change the io scheduler merge functions to get passed a pointer to
a list_head pointer instead.  This works for both inserts and merges.
In addition, deadline checks if it really should insert at the very
front.

Also don't pass in request to elv_try_last_merge(), the very name of the
function suggests that it's q->last_merge that we are interested in.