net: dst: provide accessor function to dst->xfrm

dst->xfrm is conditionally defined.  Provide accessor funtion that
is always available.
Signed-off-by: NVlad Yasevich <vyasevich@gmail.com>
Acked-by: NNeil Horman <nhorman@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/dst.h

@@ -479,10 +479,22 @@ static inline struct dst_entry *xfrm_lookup(struct net *net,
 {
 	return dst_orig;
 } 
+
+static inline struct xfrm_state *dst_xfrm(const struct dst_entry *dst)
+{
+	return NULL;
+}
+
 #else
 extern struct dst_entry *xfrm_lookup(struct net *net, struct dst_entry *dst_orig,
 				     const struct flowi *fl, struct sock *sk,
 				     int flags);
+
+/* skb attached with this dst needs transformation if dst->xfrm is valid */
+static inline struct xfrm_state *dst_xfrm(const struct dst_entry *dst)
+{
+	return dst->xfrm;
+}
 #endif
 
 #endif /* _NET_DST_H */