l2tp: use pre_exit pernet hook to avoid rcu_barrier

Move the work of closing all tunnels from the pernet exit hook to
pre_exit since the core does rcu synchronisation between these steps
and we can therefore remove rcu_barrier from l2tp code.

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>