net: add RCU annotation to sk_dst_cache field

sock->sk_dst_cache is protected by RCU.
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: NCong Wang <amwang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -340,7 +340,7 @@ struct sock {
 #endif
 	unsigned long 		sk_flags;
 	struct dst_entry	*sk_rx_dst;
-	struct dst_entry	*sk_dst_cache;
+	struct dst_entry __rcu	*sk_dst_cache;
 	spinlock_t		sk_dst_lock;
 	atomic_t		sk_wmem_alloc;
 	atomic_t		sk_omem_alloc;