xsk: Return error code if force_zc is set

If force_zc is set, we should exit out with an error, not fall back to
copy mode.

Fixes: 921b6869 ("xsk: Enable sharing of dma mappings")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NZhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NMagnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/1607077277-41995-1-git-send-email-zhangchangzhong@huawei.com

net/xdp/xsk_buff_pool.c

@@ -175,6 +175,7 @@ static int __xp_assign_dev(struct xsk_buff_pool *pool,
 
 	if (!pool->dma_pages) {
 		WARN(1, "Driver did not DMA map zero-copy buffers");
+		err = -EINVAL;
 		goto err_unreg_xsk;
 	}
 	pool->umem->zc = true;