ipip: add GSO support

In commit 6b78f16e

 (gre: add GSO support) we added GSO support to GRE
tunnels.

This patch does the same for IPIP tunnels.

Performance of single TCP flow over an IPIP tunnel is increased by 40%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Maciej Żenczykowski <maze@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>