rcutorture: Stop generic kthreads in torture_cleanup()

The specific torture modules (like rcutorture) need to call
torture_cleanup() in any case, so this commit makes torture_cleanup()
deal with torture_shutdown_cleanup() and torture_stutter_cleanup() so
that the specific modules don't have to deal with these details.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

diff --cc include/linux/torture.h
Simple merge

diff --cc kernel/rcu/rcutorture.c
Simple merge

diff --cc kernel/torture.c
Simple merge