Introduce rq_for_each_segment replacing rq_for_each_bio

Every usage of rq_for_each_bio wraps a usage of
bio_for_each_segment, so these can be combined into
rq_for_each_segment.

We define "struct req_iterator" to hold the 'bio' and 'index' that
are needed for the double iteration.

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

Various compile fixes by me...

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 964e516..6af2501 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -150,9 +150,8 @@
 	struct blkfront_info *info = req->rq_disk->private_data;
 	unsigned long buffer_mfn;
 	struct blkif_request *ring_req;
-	struct bio *bio;
+	struct req_iterator iter;
 	struct bio_vec *bvec;
-	int idx;
 	unsigned long id;
 	unsigned int fsect, lsect;
 	int ref;
@@ -186,8 +185,7 @@
 		ring_req->operation = BLKIF_OP_WRITE_BARRIER;
 
 	ring_req->nr_segments = 0;
-	rq_for_each_bio (bio, req) {
-		bio_for_each_segment (bvec, bio, idx) {
+	rq_for_each_segment(bvec, req, iter) {
 			BUG_ON(ring_req->nr_segments
 			       == BLKIF_MAX_SEGMENTS_PER_REQUEST);
 			buffer_mfn = pfn_to_mfn(page_to_pfn(bvec->bv_page));
@@ -213,7 +211,6 @@
 					.last_sect  = lsect };
 
 			ring_req->nr_segments++;
-		}
 	}
 
 	info->ring.req_prod_pvt++;