Fix regression introduced in r296986.

Currently we don't keep zoneid in in6_ifaddr structure, because there
is still some code, that doesn't properly initialize sin6_scope_id,
but some functions use sa_equal() for addresses comparison. sa_equal()
compares full sockaddr_in6 structures and such comparison will fail.
For now use zero zoneid in in6ifa_ifwithaddr(). It is safe, because
used address is in embedded form. In future we will use zoneid, so mark it
with XXX comment.

Reported by: kp
Tested by: kp