pppol2tp: Add missing sock_put() in pppol2tp_release()

pppol2tp_sock_to_session() do sock_hold() if the session to release is
not NULL.
Signed-off-by: Frédéric Moulins <frederic.moulins@alsatis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/pppol2tp.c

@@ -1353,6 +1353,7 @@ static int pppol2tp_release(struct socket *sock)
 			kfree_skb(skb);
 			sock_put(sk);
 		}
+		sock_put(sk);
 	}
 
 	release_sock(sk);