Despite the names, addr_ll_seen does not relate to addr_ll the same way
addr_see 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