block: blk_add_rq_to_plug(): clear stale 'last' after flush

blk_mq_flush_plug_list() empties ->mq_list and request we'd peeked there
before that call is gone; in any case, we are not dealing with a mix
of requests for different queues now - there's no requests left in the
plug.

Signed-off-by: Al Viro <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: Al Viro

block/blk-mq.c

@@ -1262,6 +1262,7 @@ static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
 		   (!blk_queue_nomerges(rq->q) &&
 		    blk_rq_bytes(last) >= BLK_PLUG_FLUSH_SIZE)) {
 		blk_mq_flush_plug_list(plug, false);
+		last = NULL;
 		trace_block_plug(rq->q);
 	}