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

[olce]

Summary

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This series has also been squeezed into D47633 to provide an overall view.

Commit message:
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.