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

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 <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>