dpni announces vlan_mtu but internally does not increase the maximum
MTU. Createing a vlan interface on top of a dpni interface will result
in full-sized frames not passing.
Extend the maximum frame length by ETHER_VLAN_ENCAP_LEN to allow at
least for one layer of vlans for now
MFC after: 3 days
GH-Issue: https://github.com/mcusim/freebsd-src/issues/22