rcutorture: Abstract torture_shutdown_notify()

Because handling the race between rmmod and system shutdown is not
specific to RCU, this commit abstracts torture_shutdown_notify(),
placing this code into kernel/torture.c.  This change also allows
fullstop_mutex to be private to kernel/torture.c.

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