[NETNS]: Consolidate kernel netlink socket destruction.

Create a specific helper for netlink kernel socket disposal. This just
let the code look better and provides a ground for proper disposal
inside a namespace.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Tested-by: Alexey Dobriyan <adobriyan@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index 4e5216e..e787d21 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -881,7 +881,7 @@
 	 * initial network namespace. So the socket will  be safe to free.
 	 */
 	net->ipv4.fibnl->sk_net = get_net(&init_net);
-	sock_release(net->ipv4.fibnl->sk_socket);
+	netlink_kernel_release(net->ipv4.fibnl);
 }
 
 static void fib_disable_ip(struct net_device *dev, int force)