When VIRTIO_NET_F_MTU is negotiated, virtio_net_config.mtu specifies
the maximum MTU the driver may use. vtnet_ioctl_mtu() enforces this
limit for subsequent MTU changes, but ether_ifattach() unconditionally
initializes the interface MTU to ETHERMTU (1500).
So a device advertising a maximum below 1500 starts with an MTU above
its limit. So make sure to set the initial MTU no larger than
vtnet_max_mtu. That is, for vtnet_max_mtu less than 1500
interface MTU will be set to this value, otherwise the current behaviour
is preserved, MTU will be set to 1500.
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks