block: bio: pass bvec table to bio_init()

Some drivers often use external bvec table, so introduce
this helper for this case. It is always safe to access the
bio->bi_io_vec in this way for this case.

After converting to this usage, it will becomes a bit easier
to evaluate the remaining direct access to bio->bi_io_vec,
so it can help to prepare for the following multipage bvec
support.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

Fixed up the new O_DIRECT cases.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 70acdd3..5f9e284 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -2004,13 +2004,8 @@ static struct stripe_head *alloc_stripe(struct kmem_cache *sc, gfp_t gfp,
 		for (i = 0; i < disks; i++) {
 			struct r5dev *dev = &sh->dev[i];
 
-			bio_init(&dev->req);
-			dev->req.bi_io_vec = &dev->vec;
-			dev->req.bi_max_vecs = 1;
-
-			bio_init(&dev->rreq);
-			dev->rreq.bi_io_vec = &dev->rvec;
-			dev->rreq.bi_max_vecs = 1;
+			bio_init(&dev->req, &dev->vec, 1);
+			bio_init(&dev->rreq, &dev->rvec, 1);
 		}
 	}
 	return sh;