rcutorture: Exempt tasks RCU from timely draining of grace periods

After the end of each stutter pause interval, the rcu_torture_writer()
kthread checks to be sure that all prior callbacks have completed so
that all the test structures have been freed.  This works fine except
for tasks RCU, in which grace periods can take one good long time.
This commit therefore exempts tasks RCU from this check.

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