Improve copy-and-pasted versions of SIOCGIFADDR.

The original implementation used a reference to ifr_data and a cast to
do the equivalent of accessing ifr_addr. This was copied multiple
times since 1996.

Approved by: kib
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14873