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.

---
 include/net/ipip.h |    1
 net/ipv6/sit.c     |   73 +++++++++++++++++++++++++++++--------------
 2 files changed, 51 insertions(+), 23 deletions(-)

index 86f1c8b..290effb 100644
@@ -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 {						\