MAC/do: Rename private OSD slot by removing 'mac_do_' prefix

This variable is static and holds the OSD slot number for jails that
MAC/do uses to store rules.

In the same vein as previous renames, simplify it by removing the
redundant prefix, as this name cannot appear in code outside of
'mac_do.c', nor in stack traces on panic.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47772