Share code between init_request_from_bio and blk_rq_bio_prep

These have very similar functions and should share code where
possible.

Signed-off-by: Neil Brown <neilb@suse.de>

diff .prev/block/ll_rw_blk.c ./block/ll_rw_blk.c
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>