l2tp: move l2tp_ip and l2tp_ip6 data to pernet

l2tp_ip[6] have always used global socket tables. It is therefore not
possible to create l2tpip sockets in different namespaces with the
same socket address.

To support this, move l2tpip socket tables to pernet data.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>