dm: simplify the single bio fast path in __send_duplicate_bios

Most targets just need a single flush bio.  Open code that case in
__send_duplicate_bios without the need to add the bio to a list.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Link: https://lore.kernel.org/r/20220202160109.108149-9-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/dm.c

@@ -1241,15 +1241,6 @@ static void alloc_multiple_bios(struct bio_list *blist, struct clone_info *ci,
 	struct bio *bio;
 	int try;
 
-	if (!num_bios)
-		return;
-
-	if (num_bios == 1) {
-		bio = alloc_tio(ci, ti, 0, len, GFP_NOIO);
-		bio_list_add(blist, bio);
-		return;
-	}
-
 	for (try = 0; try < 2; try++) {
 		int bio_nr;
 
@@ -1279,13 +1270,24 @@ static void __send_duplicate_bios(struct clone_info *ci, struct dm_target *ti,
 	struct bio_list blist = BIO_EMPTY_LIST;
 	struct bio *clone;
 
+	switch (num_bios) {
+	case 0:
+		break;
+	case 1:
+		clone = alloc_tio(ci, ti, 0, len, GFP_NOIO);
+		if (len)
+			bio_setup_sector(clone, ci->sector, *len);
+		__map_bio(clone);
+		break;
+	default:
 		alloc_multiple_bios(&blist, ci, ti, num_bios, len);
-
 		while ((clone = bio_list_pop(&blist))) {
 			if (len)
 				bio_setup_sector(clone, ci->sector, *len);
 			__map_bio(clone);
 		}
+		break;
+	}
 }
 
 static int __send_empty_flush(struct clone_info *ci)