fix zombie netlink socket in user space

diff -u -r1.2 af_netlink.c
@@ -306,6 +306,19 @@
 	return 0;
 }
 
+static inline int netlink_socket_exist(int protocol)
+{
+	/* Wanna bind to an non-existant netlink socket? */
+	netlink_table_grab();
+	if (!sk_head(&nl_table[protocol])) {
+		netlink_table_ungrab();
+		return 0;
+	}
+	netlink_table_ungrab();
+
+	return 1;
+}
+
 static int netlink_autobind(struct socket *sock)
 {
 	struct sock *sk = sock->sk;

@@ -351,6 +364,9 @@
 	if (nladdr->nl_family != AF_NETLINK)
 		return -EINVAL;
 
+        if (!netlink_socket_exist(sk->sk_protocol))
+		return -ENOENT;
+
 	/* Only superuser is allowed to listen multicasts */
 	if (nladdr->nl_groups && !netlink_capable(sock, NL_NONROOT_RECV))
 		return -EPERM;

@@ -392,6 +408,9 @@
 	if (addr->sa_family != AF_NETLINK)
 		return -EINVAL;
 
+        if (!netlink_socket_exist(sk->sk_protocol))
+		return -ENOENT;
+
 	/* Only superuser is allowed to send multicasts */
 	if (nladdr->nl_groups && !netlink_capable(sock, NL_NONROOT_SEND))
 		return -EPERM;