We do header check twice for a dodgy packet. One is done before
macvlan_start_xmit(), another is done before lower device's
ndo_start_xmit(). The first one seems redundant so this patch tries to
delay header check until a packet reaches its lower device (or macvtap)
through always enabling NETIF_F_GSO_ROBUST for macvlan device.

Cc: Patrick McHardy <kaber@trash.net>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>