block: fix request sorting at unplug

Comparison function for list_sort() must be anticommutative,
otherwise it is not sorting in ordinary meaning.

But fortunately list_sort() always check ((*cmp)(priv, a, b) <= 0)
it not distinguish negative and zero, so comparison function can
implement only less-or-equal instead of full three-way comparison.
Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-core.c

@@ -2665,7 +2665,7 @@ static int plug_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
 	struct request *rqa = container_of(a, struct request, queuelist);
 	struct request *rqb = container_of(b, struct request, queuelist);
 
-	return !(rqa->q == rqb->q);
+	return !(rqa->q <= rqb->q);
 }
 
 static void flush_plug_list(struct blk_plug *plug)