macvtap: simplify usage of tap_features

In macvtap, tap_features specific the features of that the user
has specified via ioctl().  If we treat macvtap as a macvlan+tap
then we could all the tap a pseudo-device and give it other features
like SG and GSO.  Then we can stop using the features of lower
device (macvlan) when forwarding the traffic the tap.

This solves the issue of possible checksum offload mismatch between
tap feature and macvlan features.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>