flow_dissector: factor out the ports extraction in skb_flow_get_ports

Factor out the code that extracts the ports from skb_flow_dissect and
add a new function skb_flow_get_ports which can be re-used.

Suggested-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/flow_keys.h b/include/net/flow_keys.h
index ac2439d..7e64bd8 100644
--- a/include/net/flow_keys.h
+++ b/include/net/flow_keys.h
@@ -14,4 +14,5 @@
 };
 
 bool skb_flow_dissect(const struct sk_buff *skb, struct flow_keys *flow);
+__be32 skb_flow_get_ports(const struct sk_buff *skb, int thoff, u8 ip_proto);
 #endif