[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/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
index 35fc16a..e0ccdf2 100644
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -2420,7 +2420,7 @@
 	xfrm_unregister_km(&netlink_mgr);
 	rcu_assign_pointer(xfrm_nl, NULL);
 	synchronize_rcu();
-	sock_release(nlsk->sk_socket);
+	netlink_kernel_release(nlsk);
 }
 
 module_init(xfrm_user_init);