Fix build on systems that lack NETLINK_SOCK_DIAG definition

NETLINK_INET_DIAG was introduced in linux v2.6.14,
NETLINK_SOCK_DIAG was added much later in linux v3.3.

* net.c [!NETLINK_SOCK_DIAG && NETLINK_INET_DIAG]
(NETLINK_SOCK_DIAG): Define.
* socketutils.c: Likewise.
* tests/netlink_unix_diag.c: Likewise.

socketutils.c

@@ -8,6 +8,10 @@
 #include <linux/unix_diag.h>
 #include <linux/rtnetlink.h>
 
+#if !defined NETLINK_SOCK_DIAG && defined NETLINK_INET_DIAG
+# define NETLINK_SOCK_DIAG NETLINK_INET_DIAG
+#endif
+
 #include <sys/un.h>
 #ifndef UNIX_PATH_MAX
 # define UNIX_PATH_MAX sizeof(((struct sockaddr_un *) 0)->sun_path)