Clean up reference management in in6_update_ifa and in6_unlink_ifa, and in particular, add a reference for in6_ifaddrhead since we do remove a reference for it when an IPv6 address is removed.  This fixes ifconfig delete of an IPv6 alias.
Clean up reference management in in6_update_ifa and in6_unlink_ifa, and
in particular, add a reference for in6_ifaddrhead since we do remove a
reference for it when an IPv6 address is removed.  This fixes ifconfig
delete of an IPv6 alias.

Reported by:	tegge
MFC after:	6 weeks

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase