rcutorture: Prepare for asynchronous access to rcu_fwd_startat

Because rcutorture's forward-progress checking will trigger from an
OOM notifier, this notifier will introduce asynchronous concurrent
access to the rcu_fwd_startat variable.  This commit therefore prepares
for this by converting updates to WRITE_ONCE().

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>