xfrm: add rcu grace period in xfrm_policy_destroy()

We will soon switch sk->sk_policy[] to RCU protection,
as SYNACK packets are sent while listener socket is not locked.

This patch simply adds RCU grace period before struct xfrm_policy
freeing, and the corresponding rcu_head in struct xfrm_policy.

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>
diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index 4a9c21f..8bae1ef 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -548,6 +548,7 @@
 	u16			family;
 	struct xfrm_sec_ctx	*security;
 	struct xfrm_tmpl       	xfrm_vec[XFRM_MAX_DEPTH];
+	struct rcu_head		rcu;
 };
 
 static inline struct net *xp_net(const struct xfrm_policy *xp)