[BLOCK] ll_rw_blk: separate out bio init part from __make_request

Separate out bio initialization part from __make_request.  It
will be used by the following blk_ordered_reimpl.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>