lagg: allow lacp to manage the link state

Lacp needs to manage the link state itself. Unlike other
lagg protocols, the ability of lacp to pass traffic
depends not only on the lagg members having link, but also
on the lacp protocol converging to a distributing state with the
link partner.

If we prematurely mark the link as up, then we will send a
gratuitous arp (via arp_handle_ifllchange()) before the lacp
interface is capable of passing traffic. When this happens,
the gratuitous arp is lost, and our link partner may cache
a stale mac address (eg, when the base mac address for the
lagg bundle changes, due to a BIOS change re-ordering NIC
unit numbers)

Reviewed by: jtl, hselasky
Sponsored by: Netflix