net: pppoe,pppol2tp - register channels with explicit net

In PPPo[E|L2TP] we could explicitly point which net namespace
we're going to use for channels - make it so.

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>