xfs_scrub_all_fail: reduce security lockdowns to avoid postfix problems

xfs_scrub_all_fail: reduce security lockdowns to avoid postfix problems

The same lockdown logic of commit 9042fcc08eed6a ("xfs_scrub_fail:
tighten up the security on the background systemd service") was applied
to the media scan failure reporting service.  Therefore, it's also
broken on systems that have setgid mailer programs (e.g. postfix).  Fix
this by applying the same change from commit 15fd6fc686d5ce here too.

While we're at it, put this service in the background scrub slice for
resource control.

Cc: linux-xfs@vger.kernel.org # v6.17.0
Fixes: 15fd6fc686d5ce ("xfs_scrub_fail: reduce security lockdowns to avoid postfix problems")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Fixes: e040916f649f ("xfs_scrub_all: failure reporting for the xfs_scrub_all job")
Reviewed-by: Andrey Albershteyn <aalbersh@kernel.org>