macvtap: Perform GSO on forwarding path.

When macvtap forwards skb to its tap, it needs to check
if GSO needs to be performed.  This is sometimes necessary
when the HW device performed GRO, but the guest reading
from the tap does not support it (ex: Windows 7).

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