ixgbe: Look inside vlan when determining offload protocol.

Currently the skb->protocol field is used to setup various
offloading parameters on transmit for the correct protocol.
However, if vlan offloading is disabled or otherwise not used,
the protocol field will be ETH_P_8021Q, not the actual protocol.
This will cause the offloading to be not performed correctly,
even though the hardware is capable of looking inside vlan tags.
Instead, look inside the header if necessary to determine the
correct protocol type.

To some extent this fixes a regression from 2.6.36 because it
was previously not possible to disable vlan offloading and this
error case was not exposed.

Signed-off-by: Hao Zheng <hzheng@nicira.com>
CC: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
CC: Alex Duyck <alexander.h.duyck@intel.com>
CC: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>