macvtap: signal truncated packets

macvtap_put_user() never return a value grater than iov length, this in fact
bypasses the truncated checking in macvtap_recvmsg(). Fix this by always
returning the size of packet plus the possible vlan header to let the trunca
checking work.

Cc: Vlad Yasevich <vyasevich@gmail.com>
Cc: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>