l2tp: push all ppp pseudowire shutdown through .release handler

If userspace deletes a ppp pseudowire using the netlink API, either by
directly deleting the session or by deleting the tunnel that contains the
session, we need to tear down the corresponding pppox channel.

Rather than trying to manage two pppox unbind codepaths, switch the netlink
and l2tp_core session_close handlers to close via. the l2tp_ppp socket
.release handler.

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>