macvtap: Ignore tap features when VNET_HDR is off

When the user turns off VNET_HDR support on the
macvtap device, there is no way to provide any
offload information to the user.  So, it's safer
to ignore offload setting then depend on the user
setting them correctly.

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