l2tp: set netnsok flag for netlink messages

The L2TP netlink code can run in namespaces.  Set the netnsok flag in
genl_family to true to reflect that fact.

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