[ Upstream commit 99469c32 ]

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>