md/raid0: convert raid0_handle_discard() to use bio_submit_split_bioset()

Unify bio split code, and prepare to fix ordering of split IO

Noted commit 319ff40a5427 ("md/raid0: Fix performance regression for large
sequential writes") already fix ordering of split IO by remapping bio to
underlying disks before resubmitting it, with the respect
md_submit_bio() already split it by sectors, and raid0_make_request()
will split at most once for unaligned IO. This is a bit hacky and we'll
convert this to solution in general later.

Signed-off-by: Yu Kuai <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/md/raid0.c

···
 	zone = find_zone(conf, &start);
 
 	if (bio_end_sector(bio) > zone->zone_end) {
-		struct bio *split = bio_split(bio,
-			zone->zone_end - bio->bi_iter.bi_sector, GFP_NOIO,
-			&mddev->bio_set);
-
-		if (IS_ERR(split)) {
-			bio->bi_status = errno_to_blk_status(PTR_ERR(split));
-			bio_endio(bio);
+		bio = bio_submit_split_bioset(bio,
+				zone->zone_end - bio->bi_iter.bi_sector,
+				&mddev->bio_set);
+		if (!bio)
 			return;
-		}
 
-		bio_chain(split, bio);
-		trace_block_split(split, bio->bi_iter.bi_sector);
-		submit_bio_noacct(bio);
-		bio = split;
 		end = zone->zone_end;
-	} else
+	} else {
 		end = bio_end_sector(bio);
+	}
 
 	orig_end = end;
 	if (zone != conf->strip_zone)