samples/bpf: Fix the error return code of xdp_redirect's main()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

If bpf_map_update_elem() failed, main() should return a negative error.

Fixes: 832622e6bd ("xdp: sample program for new bpf_redirect helper")
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210616042534.315097-1-wanghai38@huawei.com

samples/bpf/xdp_redirect_user.c

@@ -213,5 +213,5 @@ int main(int argc, char **argv)
 	poll_stats(2, ifindex_out);
 
 out:
-	return 0;
+	return ret;
 }