rcutorture: Adjust rcu_poll_need_2gp() for rcu_gp_oldstate field removal

Now that rcu_gp_oldstate can accurately track both normal and
expedited grace periods regardless of system state, rcutorture's
rcu_poll_need_2gp() function need only call for a second grace period
for the old single-unsigned-long grace-period polling APIs
This commit therefore adjusts rcu_poll_need_2gp() accordingly.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>