geneve: Set GSO type on transmit.

Geneve does not currently set the inner protocol type when
transmitting packets. This causes GSO segmentation to fail on NICs
that do not support Geneve offloading.

CC: Andy Zhou <azhou@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>