block: ensure bio_alloc_map_data() deals with ITER_UBUF correctly

This helper blindly copies the iovec, even if we don't have one.
Make this case a bit smarter by only doing so if we have an iovec
array to copy.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-map.c

@@ -29,10 +29,11 @@ static struct bio_map_data *bio_alloc_map_data(struct iov_iter *data,
 	bmd = kmalloc(struct_size(bmd, iov, data->nr_segs), gfp_mask);
 	if (!bmd)
 		return NULL;
-	memcpy(bmd->iov, data->iov, sizeof(struct iovec) * data->nr_segs);
 	bmd->iter = *data;
-	if (iter_is_iovec(data))
+	if (iter_is_iovec(data)) {
+		memcpy(bmd->iov, data->iov, sizeof(struct iovec) * data->nr_segs);
 		bmd->iter.iov = bmd->iov;
+	}
 	return bmd;
 }