net: pppol2tp - introduce net-namespace functionality

- Each tunnel and appropriate lock are inside own namespace now.
- pppox code allows to create per-namespace sockets for
  both PX_PROTO_OE and PX_PROTO_OL2TP protocols. Actually since
  now pppox_create support net-namespaces new PPPo... protocols
  (if they ever will be) should support net-namespace too otherwise
  explicit check for &init_net would be needed.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>