refscale: Ditch ref_scale_shutdown in favor of torture_shutdown_init()

The torture_shutdown_init() function spawns a shutdown kthread in
a manner very similar to that implemented by ref_scale_shutdown().
This commit therefore re-implements ref_scale_shutdown in terms of
torture_shutdown_init().

The initial draft of this patch was generated by version 2.1.16 of the
Claude AI/LLM, but trained and configured for use by my employer, and
prompted to refer to Linux-kernel source code.  This initial draft failed
to provide a forward reference to ref_scale_cleanup(), passed zero to
torture_shutdown_init() for an unwelcome insta-shutdown, and failed to
pass the kvm.sh --duration argument in as a refscale module parameter.
On the other hand, it did catch the need to NULL main_task on the
post-test self-shutdown code path, which I might well have forgotten
to do.

This version of the patch fixes those problems, and in fact very little
of the initial draft remains.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

tools/testing/selftests/rcutorture/configs/refscale/ver_functions.sh

@@ -11,7 +11,7 @@
 #
 # Adds per-version torture-module parameters to kernels supporting them.
 per_version_boot_params () {
-	echo	refscale.shutdown=1 \
+	echo	refscale.shutdown_secs=$3 \
 		refscale.verbose=0 \
 		$1
 }