macvtap: Let TUNSETOFFLOAD actually controll offload features.

When the user issues TUNSETOFFLOAD ioctl, macvtap does not do
anything other then to verify arguments.  This patch adds
functionality to allow users to actually control offload features.
NETIF_F_GSO and NETIF_F_GRO are always on, but the rest of the
features can be controlled.

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