block: deal with stale req count of plug list

In both legacy and mq path, req count of plug list is computed
before allocating request, so the number can be stale when falling
back to slept allocation, also the new introduced wbt can sleep
too.

This patch deals with the case by checking if plug list becomes
empty, and fixes the KASAN report of 'BUG: KASAN: stack-out-of-bounds'
which is introduced by Shaohua's patches of dispatching big request.

Fixes: 600271d900002(blk-mq: immediately dispatch big size request)
Fixes: 50d24c34403c6(block: immediately dispatch big size request)
Cc: Shaohua Li <shli@fb.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-core.c

@@ -1753,8 +1753,11 @@ get_rq:
 		/*
 		 * If this is the first request added after a plug, fire
 		 * of a plug trace.
+		 *
+		 * @request_count may become stale because of schedule
+		 * out, so check plug list again.
 		 */
-		if (!request_count)
+		if (!request_count || list_empty(&plug->list))
 			trace_block_plug(q);
 		else {
 			struct request *last = list_entry_rq(plug->list.prev);

block/blk-mq.c

@@ -1497,6 +1497,13 @@ static blk_qc_t blk_sq_make_request(struct request_queue *q, struct bio *bio)
 		struct request *last = NULL;
 
 		blk_mq_bio_to_request(rq, bio);
+
+		/*
+		 * @request_count may become stale because of schedule
+		 * out, so check the list again.
+		 */
+		if (list_empty(&plug->mq_list))
+			request_count = 0;
 		if (!request_count)
 			trace_block_plug(q);
 		else