selftests: net: reuseport_dualstack: fix uninitalized parameter

This test reports EINVAL for getsockopt(SOL_SOCKET, SO_DOMAIN)
occasionally due to the uninitialized length parameter.
Initialize it to fix this, and also use int for "test_family" to comply
with the API standard.

Fixes: d6a61f8 ("soreuseport: test mixed v4/v6 sockets")
Reported-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Wei Wang <weiwan@google.com>
Cc: Craig Gallek <cgallek@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/reuseport_dualstack.c

@@ -129,7 +129,7 @@ static void test(int *rcv_fds, int count, int proto)
 {
 	struct epoll_event ev;
 	int epfd, i, test_fd;
-	uint16_t test_family;
+	int test_family;
 	socklen_t len;
 
 	epfd = epoll_create(1);
@@ -146,6 +146,7 @@ static void test(int *rcv_fds, int count, int proto)
 	send_from_v4(proto);
 
 	test_fd = receive_once(epfd, proto);
+	len = sizeof(test_family);
 	if (getsockopt(test_fd, SOL_SOCKET, SO_DOMAIN, &test_family, &len))
 		error(1, errno, "failed to read socket domain");
 	if (test_family != AF_INET)