xfrm: add rcu protection to sk->sk_policy[]

XFRM can deal with SYNACK messages, sent while listener socket
is not locked. We add proper rcu protection to __xfrm_sk_clone_policy()
and xfrm_sk_policy_lookup()

This might serve as the first step to remove xfrm.xfrm_policy_lock
use in fast path.

Fixes: fa76ce7328 ("inet: get rid of central tcp/dccp listener timer")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -388,7 +388,7 @@ struct sock {
 		struct socket_wq	*sk_wq_raw;
 	};
 #ifdef CONFIG_XFRM
-	struct xfrm_policy	*sk_policy[2];
+	struct xfrm_policy __rcu *sk_policy[2];
 #endif
 	struct dst_entry	*sk_rx_dst;
 	struct dst_entry __rcu	*sk_dst_cache;