blk-mq: don't plug for head insertions in blk_execute_rq_nowait

Plugs never insert at head, so don't plug for head insertions.

Fixes: 1c2d2fff ("block: wire-up support for passthrough plugging")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20230413064057.707578-2-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -1299,7 +1299,7 @@ void blk_execute_rq_nowait(struct request *rq, bool at_head)
 	 * device, directly accessing the plug instead of using blk_mq_plug()
 	 * should not have any consequences.
 	 */
-	if (current->plug)
+	if (current->plug && !at_head)
 		blk_add_rq_to_plug(current->plug, rq);
 	else
 		blk_mq_sched_insert_request(rq, at_head, true, false);