[BLOCK] Don't clear sg_dma_len/addr() in blk_rq_map_sg()

It's not a proper lvalue on all archs.
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

block/ll_rw_blk.c

@@ -1354,8 +1354,6 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
 			else
 				sg = sg_next(sg);
 
-			sg_dma_len(sg) = 0;
-			sg_dma_address(sg) = 0;
 			sg_set_page(sg, bvec->bv_page);
 			sg->length = nbytes;
 			sg->offset = bvec->bv_offset;