flow_keys: include thoff into flow_keys for later usage

In skb_flow_dissect(), we perform a dissection of a skbuff. Since we're
doing the work here anyway, also store thoff for a later usage, e.g. in
the BPF filter.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 9d4c720..e187bf0 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -140,6 +140,8 @@
 			flow->ports = *ports;
 	}
 
+	flow->thoff = (u16) nhoff;
+
 	return true;
 }
 EXPORT_SYMBOL(skb_flow_dissect);