auditon(2): fix A_SETPOLICY with 64-bit values

A_SETPOLICY is supposed to work with either 64 or 32-bit values, but due to a
typo the 64-bit version has never worked correctly.

Submitted by: aniketp
Reviewed by: asomers, cem
MFC after: 2 weeks
Sponsored by: Google, Inc. (GSoC 2018)
Differential Revision: https://reviews.freebsd.org/D16222