Fix access to ifru_buffer on freebsd32.

Make all kernel accesses to ifru_buffer go via access functions
which take the process ABI into account and use an appropriate union
to access members in the correct place in struct ifreq.

Reviewed by: kib
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14846