xfrm: Add a secpath_set helper.

Add a new helper to set the secpath to the skb.
This avoids code duplication, as this is used
in multiple places.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index 6e06130..287635d 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -1006,6 +1006,7 @@
 }
 
 struct sec_path *secpath_dup(struct sec_path *src);
+int secpath_set(struct sk_buff *skb);
 
 static inline void
 secpath_reset(struct sk_buff *skb)