ipip: Properly mark ipip GRO packets as encapsulated.

ipip encapsulated packets can be merged together by GRO but the result
does not have the proper GSO type set or even marked as being
encapsulated at all. Later retransmission of these packets will likely
fail if the device does not support ipip offloads. This is similar to
the issue resolved in IPv6 sit in feec0cb3
("ipv6: gro: support sit protocol").

Reported-by: Patrick Boutilier <boutilpj@ednet.ns.ca>
Fixes: 9667e9bb ("ipip: Add gro callbacks to ipip offload")
Tested-by: Patrick Boutilier <boutilpj@ednet.ns.ca>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jesse Gross <jesse@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>