net: tso: add support for IPv6

Adding IPv6 for the TSO helper API is trivial:
* Don't play with the id (which doesn't exist in IPv6)
* Correctly update the payload_len (don't include the
  length of the IP header itself)

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tso.h

@@ -8,6 +8,7 @@ struct tso_t {
 	void *data;
 	size_t size;
 	u16 ip_id;
+	bool ipv6;
 	u32 tcp_seq;
 };