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>

include/net/flow_keys.h

@@ -9,6 +9,7 @@ struct flow_keys {
 		__be32 ports;
 		__be16 port16[2];
 	};
+	u16 thoff;
 	u8 ip_proto;
 };