msdosfs_fat: reduce diffs with NetBSD and makefs

Use pointer arithmetic (as now done in makefs, and in NetBSD) instead of
taking the address of array element. No functional change, but this
makes it easier to compare different versions of this file.

Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21365