drivers/net/pppol2tp.c: remove exceptional & on function name

In this file, function names are otherwise used as pointers without &.

A simplified version of the semantic patch that makes this change is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@r@
identifier f;
@@

f(...) { ... }

@@
identifier r.f;
@@

- &f
+ f
// </smpl>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/pppol2tp.c

@@ -1537,7 +1537,7 @@ static struct sock *pppol2tp_prepare_tunnel_socket(struct net *net,
 	 * if the tunnel socket goes away.
 	 */
 	tunnel->old_sk_destruct = sk->sk_destruct;
-	sk->sk_destruct = &pppol2tp_tunnel_destruct;
+	sk->sk_destruct = pppol2tp_tunnel_destruct;
 
 	tunnel->sock = sk;
 	sk->sk_allocation = GFP_ATOMIC;