flowi: Abstract out functions to get flow hash based on flowi

Create __get_hash_from_flowi6 and __get_hash_from_flowi4 to get the
flow keys and hash based on flowi structures. These are called by
__skb_get_hash_flowi6 and __skb_get_hash_flowi4. Also, created
get_hash_from_flowi6 and get_hash_from_flowi4 which can be called
when just the hash value for a flowi is needed.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/flow_dissector.h b/include/net/flow_dissector.h
index af76c49..74fe160 100644
--- a/include/net/flow_dissector.h
+++ b/include/net/flow_dissector.h
@@ -172,4 +172,6 @@
 	return (keys->ports.ports || keys->tags.flow_label);
 }
 
+u32 flow_hash_from_keys(struct flow_keys *keys);
+
 #endif