macvtap: fix network header pointer for VLAN tagged pkts

Network header is set with offset ETH_HLEN but it is not true for VLAN
(multiple-)tagged and results in checksum issues in lower devices.

v2: leave skb->protocol untouched (thx Vlad), comment added
v3: moved after skb_probe_transport_header() call (thx Toshiaki)

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>