tunnels: add _rcu annotations

(struct ip6_tnl)->next is rcu protected :
(struct ip_tunnel)->next is rcu protected :
(struct xfrm6_tunnel)->next is rcu protected :

add __rcu annotation and proper rcu primitives.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/ipip.h b/include/net/ipip.h
index 58abbf9..0403fe4 100644
--- a/include/net/ipip.h
+++ b/include/net/ipip.h
@@ -16,7 +16,7 @@
 };
 
 struct ip_tunnel {
-	struct ip_tunnel	*next;
+	struct ip_tunnel __rcu	*next;
 	struct net_device	*dev;
 
 	int			err_count;	/* Number of arrived ICMP errors */