Despite the names, addr_ll_seen does not relate to addr_ll the same
way addr_seen relates to addr. addr_ll_seen is an observed address
from the guest, whereas addr_ll is *our* link-local address for use on
the tap link when we can't use an external endpoint address. It's
used both for passt provided services (DHCPv6, NDP) and in some cases
for connections from addresses the guest can't access.
Signed-off-by: David Gibson