drbd: remove drbd_req_make_private_bio

Open code drbd_req_make_private_bio in the two callers to prepare
for further changes.  Also don't bother to initialize bi_next as the
bio code already does that that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Acked-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>