Possibly do some entra work in case we would not get into the
ifa0 != NULL paths later as we already do for the mltaddr before.
XXX We should possibly error in case in6_setscope fails.
Reference: http://lists.freebsd.org/pipermail/freebsd-net/2011-September/029829.html
Submitted by: bz
MFC after: 1 week