netlink: pass the correct arguments for SIOCDIFADDR and SIOCDIFADDR_IN6

These take struct ifreq and struct in6_ifreq respectively. Passing struct
in_aliasreq or struct in6_aliasreq means we're supplying a shorter object than
expected. While this doesn't actively break things on most architectures other
than CHERI it is still wrong.

Reported by: CheriBSD
Event: Kitchener-Waterloo Hackathon 202406