block: avoid to merge splitted bio

The splitted bio has been already too fat to merge, so mark it
as NOMERGE.
Reviewed-by: NJeff Moyer <jmoyer@redhat.com>
Signed-off-by: NMing Lei <ming.lei@canonical.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

block/blk-merge.c

@@ -139,6 +139,9 @@ void blk_queue_split(struct request_queue *q, struct bio **bio,
 	bio_set_flag(res, BIO_SEG_VALID);
 
 	if (split) {
+		/* there isn't chance to merge the splitted bio */
+		split->bi_rw |= REQ_NOMERGE;
+
 		bio_chain(split, *bio);
 		generic_make_request(*bio);
 		*bio = split;