net: add RCU annotation to sk_dst_cache field

sock->sk_dst_cache is protected by RCU.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/sock.h b/include/net/sock.h
index 581dc6b..a340ab4 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -340,7 +340,7 @@
 #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;