bonding: Advertize vxlan offload features when supported

When the underlying device supports TCP offloads for VXLAN/UDP
encapulated traffic, we need to reflect that through the hw_enc_features
field of the bonding net-device. This will cause the xmit path
in the core networking stack to provide bonding with encapsulated
GSO frames to offload into the HW etc.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>