net/enic: enable overlay offload for VXLAN and GENEVE

net/enic: enable overlay offload for VXLAN and GENEVE

Recent NIC models support overlay offload. The overlay offload
feature enables the following on the NIC.
- Rx/Tx checksum offloads for both inner and outer packets.
- Rx inner packet type classification.
- TSO.
- Inner RSS.

TX descriptors do not require any changes, except the header length
for TSO. The NIC parses outer/inner packets and performs offloads on
them as necessary. The header length for tunneled TSO includes both
inner and outer headers.

The NIC actually parses and performs the above for NVGRE as well. DPDK
currently has no offload flags for NVGRE, and the hardware has no
controls to individually enable tunnel types either. So do nothing for
now.

The driver enables overlay offload by default. Add a devargs
'disable-overlay=<0|1>' to allow the app to disable it.

Also update the enic guide doc.

Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com>
Reviewed-by: John Daley <johndale@cisco.com>