l2tp: avoid to use synchronize_rcu in tunnel free function

[ Upstream commit 99469c32f79a32d8481f87be0d3c66dad286f4ec ]

Avoid to use synchronize_rcu in l2tp_tunnel_free because context may be
atomic.

Signed-off-by: Dmitry Kozlov <xeb@mail.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>