makefs: make buf generic

it has nothing to do with ffs and will eventually be moved.
gc sectorsize.

This is a corrected version of r317744.

NetBSD versions:
ffs.c 1.58
ffs/buf.c 1.14 1.18
ffs/buf.h 1.8

Submitted by: Siva Mahadevan <smahadevan@freebsdfoundation.org>
Obtained from: NetBSD
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10803