Remove {max/min}_offset() macros, use vm_map_{max/min}() inlines.

Exposing max_offset and min_offset defines in public headers is
causing clashes with variable names, for example when building QEMU.

Based on the submission by: royger
Reviewed by: alc, markj (previous version)
Sponsored by: The FreeBSD Foundation (kib)
MFC after: 1 week
Approved by: re (marius)
Differential revision: https://reviews.freebsd.org/D16881