ipv6 sit: RCU conversion phase I

SIT tunnels use one rwlock to protect their prl entries.

This first patch adds RCU locking for prl management,
with standard call_rcu() calls.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ipip.h

@@ -45,6 +45,7 @@ struct ip_tunnel_prl_entry
 	struct ip_tunnel_prl_entry	*next;
 	__be32				addr;
 	u16				flags;
+	struct rcu_head			rcu_head;
 };
 
 #define IPTUNNEL_XMIT() do {						\