handle packets up to 1536 bytes)
This fixes the need to frag that could happen when using vlans on top of
if_arge (which is a common case for the use the switch ports as individual
NICs).
Previously to this commit any vlan setup with if_arge as parent would have
the MTU of the parent interface reduced by the size of dot1q header
(4 bytes).
Tested on TP-Link 1043ND (where the WAN port is just a switch port setup to
tag packets in a different VLAN than the LAN ports).
Reported and tested by: Harm Weites (harm at weites.com)
b8da8e9a79