ifconf(): Always zero the whole struct ifreq.

The previous split of zeroing ifr_name and ifr_addr seperately is safe
on current architectures, but would be unsafe if pointers were larger
than 8 bytes. Combining the zeroing adds no real cost (a few
instructions) and makes the security property easier to verify.

Reviewed by: kib, emaste
Obtained from: CheriBSD
MFC after: 3 days
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14912