l2tp: add identifier name in function pointer prototype

Reported by checkpatch:

        "WARNING: function definition argument 'struct sock *'
         should also have an identifier name"

Add an identifier name to help document the prototype.
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.h

@@ -154,7 +154,7 @@ struct l2tp_tunnel {
 	struct net		*l2tp_net;	/* the net we belong to */
 
 	refcount_t		ref_count;
-	void (*old_sk_destruct)(struct sock *);
+	void (*old_sk_destruct)(struct sock *sk);
 	struct sock		*sock;		/* parent socket */
 	int			fd;		/* parent fd, if tunnel socket was created
 						 * by userspace