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/ipip.h b/include/net/ipip.h
index 0403fe4..a32654d 100644
--- a/include/net/ipip.h
+++ b/include/net/ipip.h
@@ -34,12 +34,12 @@
 #ifdef CONFIG_IPV6_SIT_6RD
 	struct ip_tunnel_6rd_parm	ip6rd;
 #endif
-	struct ip_tunnel_prl_entry	*prl;		/* potential router list */
+	struct ip_tunnel_prl_entry __rcu *prl;		/* potential router list */
 	unsigned int			prl_count;	/* # of entries in PRL */
 };
 
 struct ip_tunnel_prl_entry {
-	struct ip_tunnel_prl_entry	*next;
+	struct ip_tunnel_prl_entry __rcu *next;
 	__be32				addr;
 	u16				flags;
 	struct rcu_head			rcu_head;