bio_copy_user_iov(): don't ignore ->iov_offset

Since "block: support large requests in blk_rq_map_user_iov" we
started to call it with partially drained iter; that works fine
on the write side, but reads create a copy of iter for completion
time.  And that needs to take the possibility of ->iov_iter != 0
into account...

Cc: stable@vger.kernel.org #v4.5+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

block/bio.c

@@ -1239,8 +1239,8 @@ struct bio *bio_copy_user_iov(struct request_queue *q,
 	 */
 	bmd->is_our_pages = map_data ? 0 : 1;
 	memcpy(bmd->iov, iter->iov, sizeof(struct iovec) * iter->nr_segs);
-	iov_iter_init(&bmd->iter, iter->type, bmd->iov,
-			iter->nr_segs, iter->count);
+	bmd->iter = *iter;
+	bmd->iter.iov = bmd->iov;
 
 	ret = -ENOMEM;
 	bio = bio_kmalloc(gfp_mask, nr_pages);