md/raid10: convert read/write to use bio_submit_split_bioset()

md/raid10: convert read/write to use bio_submit_split_bioset()

Unify bio split code, prepare to fix ordering of split IO, the error path
is modified a bit, however no functional changes are intended:

- bio_submit_split_bioset() can fail the original bio directly
  by split error, set R10BIO_Uptodate in this case to notify
  raid_end_bio_io() that the original bio is returned already.
- set R10BIO_Uptodate and set error value to -EIO is useless now,
  for r10_bio without R10BIO_Uptodate, -EIO will be returned for
  original bio.

And discard is not handled, because discard is only split for
unaligned head and tail, and this can be considered slow path, the
reorder here does not matter much.

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