i40e: support double vlan stripping and insertion

i40e: support double vlan stripping and insertion

It configures specific registers to enable double vlan stripping
on RX side and insertion on TX side.
The RX descriptors will be parsed, the vlan tags and flags will be
saved to corresponding mbuf fields if vlan tag is detected.
The TX descriptors will be configured according to the
configurations in mbufs, to trigger the hardware insertion of
double vlan tags for each packets sent out.

Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>