net: Add skb_get_hash_perturb

This calls flow_disect and __skb_get_hash to procure a hash for a
packet. Input includes a key to initialize jhash. This function
does not set skb->hash.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -927,6 +927,8 @@ static inline __u32 skb_get_hash(struct sk_buff *skb)
 	return skb->hash;
 }
 
+__u32 skb_get_hash_perturb(const struct sk_buff *skb, u32 perturb);
+
 static inline __u32 skb_get_hash_raw(const struct sk_buff *skb)
 {
 	return skb->hash;