Some nic drivers (including iflib) do not initialize if_hwassist until after the interface is brought up.
If a lagg member is included in a lagg when its not yet been brought up, that will cause lagg to see
if_hwassist=0 and will disable all checksum offload, etc, on the interface. This is almost impossible
to debug without kgdb or dtrace, as ifconfig does not surface if_hwassist.
Fix this by re-calculating lagg caps (including if_hwassist) after adding a port.
I encountered this problem when I had a commented-out if_foo1=up entry in my rc.conf that i neglected
to uncomment when I was re-configuring a lagg.