l2tp: fix memory allocation

Since .size is set properly in "struct pernet_operations l2tp_net_ops",
allocating space for "struct l2tp_net" by hand is not correct, even causes
memory leakage.

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>