tunnels: add __rcu annotations

Add __rcu annotations to :
        (struct ip_tunnel)->prl
        (struct ip_tunnel_prl_entry)->next
        (struct xfrm_tunnel)->next
	struct xfrm_tunnel *tunnel4_handlers
	struct xfrm_tunnel *tunnel64_handlers

And use appropriate rcu primitives to reduce sparse warnings if
CONFIG_SPARSE_RCU_POINTER=y

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index ffcd478..bcfb6b2 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -1264,7 +1264,7 @@
 	int (*handler)(struct sk_buff *skb);
 	int (*err_handler)(struct sk_buff *skb, u32 info);
 
-	struct xfrm_tunnel *next;
+	struct xfrm_tunnel __rcu *next;
 	int priority;
 };