fattime: make the test code build again

This change...

1. replaces calls to timet2fattime/fattime2timet with calls to timespec2fattime/fattime2timespec. The functions got renamed shortly after they landed in the kernel but the test code wasn't updated (see 7ea93e912bf0ef).
2. adds a utc_offset stub.

With this, the test code builds and runs as a 32-bit binary (cc -Wall -O2
-m32 subr_fattime.c).

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44753