Not sure the new bio flag is really required for this case, just wondering
why not take the following simple way? BTW we are really going to run
out of bio flag.

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 55bcee5dc032..1139b1efd712 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -2157,6 +2157,7 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
 	unsigned int nr_segs;
 	blk_qc_t cookie;
 	blk_status_t ret;
+	struct bio *orig_bio = bio;
 
 	blk_queue_bounce(q, &bio);
 	__blk_queue_split(&bio, &nr_segs);

@@ -2265,6 +2266,10 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
 		blk_mq_sched_insert_request(rq, false, true, true);
 	}
 
+	/* don't poll splitted bio */
+	if (orig_bio != bio)
+		return BLK_QC_T_NONE;
+
 	return cookie;
 queue_exit:
 	blk_queue_exit(q);