macvlan: add offload features for encapsulation

Currently macvlan devices do not set their hw_enc_features making
encapsulated Tx packets resort to SW fallbacks. Add encapsulation GSO
offloads to ->features as is done for the other GSOs and set
->hw_enc_features.

Signed-off-by: Dimitris Michailidis <dmichail@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>