[IPSEC]: Add xfrm_input_state helper

This patch adds the xfrm_input_state helper function which returns the
current xfrm state being processed on the input path given an sk_buff.
This is currently only used by xfrm_input but will be used by ESP upon
asynchronous resumption.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/xfrm.h

@@ -1317,4 +1317,9 @@ static inline void xfrm_states_delete(struct xfrm_state **states, int n)
 }
 #endif
 
+static inline struct xfrm_state *xfrm_input_state(struct sk_buff *skb)
+{
+	return skb->sp->xvec[skb->sp->len - 1];
+}
+
 #endif	/* _NET_XFRM_H */

net/xfrm/xfrm_input.c

@@ -109,7 +109,7 @@ int xfrm_input(struct sk_buff *skb, int nexthdr, __be32 spi, int encap_type)
 	/* A negative encap_type indicates async resumption. */
 	if (encap_type < 0) {
 		async = 1;
-		x = skb->sp->xvec[skb->sp->len - 1];
+		x = xfrm_input_state(skb);
 		seq = XFRM_SKB_CB(skb)->seq;
 		goto resume;
 	}