ipv4: add __rcu annotations to routes.c

Add __rcu annotations to :
        (struct dst_entry)->rt_next
        (struct rt_hash_bucket)->chain

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/dst.h b/include/net/dst.h
index a217c83..ffe9cb7 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -95,7 +95,7 @@
 	unsigned long		lastuse;
 	union {
 		struct dst_entry *next;
-		struct rtable    *rt_next;
+		struct rtable __rcu *rt_next;
 		struct rt6_info   *rt6_next;
 		struct dn_route  *dn_next;
 	};