block: add a req_bvec helper

Return the currently active bvec segment, potentially spanning multiple
pages.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>

include/linux/blkdev.h

@@ -932,6 +932,17 @@ static inline unsigned int blk_rq_payload_bytes(struct request *rq)
 	return blk_rq_bytes(rq);
 }
 
+/*
+ * Return the first full biovec in the request.  The caller needs to check that
+ * there are any bvecs before calling this helper.
+ */
+static inline struct bio_vec req_bvec(struct request *rq)
+{
+	if (rq->rq_flags & RQF_SPECIAL_PAYLOAD)
+		return rq->special_vec;
+	return mp_bvec_iter_bvec(rq->bio->bi_io_vec, rq->bio->bi_iter);
+}
+
 static inline unsigned int blk_queue_get_max_sectors(struct request_queue *q,
 						     int op)
 {