netlink: create a new header for internal genetlink symbols

There are things in linux/genetlink.h which are only used
under net/netlink/. Move them to a new local header.
A new header with just 2 externs isn't great, but alternative
would be to include af_netlink.h in genetlink.c which feels
even worse.

Link: https://lore.kernel.org/r/20240329175710.291749-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/genetlink.h

@@ -4,15 +4,10 @@
 
 #include <uapi/linux/genetlink.h>
 
-
 /* All generic netlink requests are serialized by a global lock.  */
 extern void genl_lock(void);
 extern void genl_unlock(void);
 
-/* for synchronisation between af_netlink and genetlink */
-extern atomic_t genl_sk_destructing_cnt;
-extern wait_queue_head_t genl_sk_destructing_waitq;
-
 #define MODULE_ALIAS_GENL_FAMILY(family)\
  MODULE_ALIAS_NET_PF_PROTO_NAME(PF_NETLINK, NETLINK_GENERIC, "-family-" family)
 

net/netlink/af_netlink.c

@@ -59,7 +59,6 @@
 #include <linux/rhashtable.h>
 #include <asm/cacheflush.h>
 #include <linux/hash.h>
-#include <linux/genetlink.h>
 #include <linux/net_namespace.h>
 #include <linux/nospec.h>
 #include <linux/btf_ids.h>
@@ -73,6 +72,7 @@
 #include <trace/events/netlink.h>
 
 #include "af_netlink.h"
+#include "genetlink.h"
 
 struct listeners {
 	struct rcu_head		rcu;

net/netlink/genetlink.c

@@ -22,6 +22,8 @@
 #include <net/sock.h>
 #include <net/genetlink.h>
 
+#include "genetlink.h"
+
 static DEFINE_MUTEX(genl_mutex); /* serialization of message processing */
 static DECLARE_RWSEM(cb_lock);
 

net/netlink/genetlink.h

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __NET_GENETLINK_H
+#define __NET_GENETLINK_H
+
+#include <linux/wait.h>
+
+/* for synchronisation between af_netlink and genetlink */
+extern atomic_t genl_sk_destructing_cnt;
+extern wait_queue_head_t genl_sk_destructing_waitq;
+
+#endif	/* __LINUX_GENERIC_NETLINK_H */