Move SIOCAIFADDR_IN6 (current "primary" ioctl to add an IPv6 interface address) handling code to the dedicated in6_addifaddr() function and make it a part of KPI. This allows in-kernel users to add/delete interfaces addresses without relying on ioctl interface.

Note: the code in the function is a full copy-paste, except 2 places:

• the first difference is "compat" handling for CARP - instead of comparing the CMDs, the ioctl handler now zeroes vhid value so the carp check always fails for compat cmd.
• the second is the return in the furst in6ifa_ifpwithaddr() check - as ia is NULL, it is possible to simply return with success.

The function by itself consumes passed ia.