enable the full use of jumbos and remove IFCAP_VLAN_MTU.
The chip can do 9008 byte packets (not including the ethernet
header), but only 9004 if you want to enable IFCAP_VLAN_MTU. by not
enabling IFCAP_VLAN_MTU, we let other protocols (eg, mpls or svlan)
use the extra bytes if they want.
using the extra bytes for the hardmtu instead of for IFCAP_VLAN_MTU
works a bit better with how aggr(4) is set up at the moment because
aggr does not pass IFCAP_VLAN_MTU through from its ports, which
means vlan(4) on aggr(4) cannot see the flag and use the extra
this was figured out by hrvoje popovski in a discussion with pedro
caetano on the "issues configuring vlan on top of aggr device" on
hrvoje also tested the diff and made sure the full use of jumbos
works for things like ping packets with DF set.
jmatthew skimmed the diff and didnt see anything obviously wrong too