rcutorture: Verify NUM_ACTIVE_RCU_POLL_FULL_OLDSTATE

This commit adds code to the RTWS_POLL_GET_FULL case
of rcu_torture_writer() to verify that the value of
NUM_ACTIVE_RCU_POLL_FULL_OLDSTATE is sufficiently large.

[ paulmck: Fix whitespace issue located by checkpatch.pl. ]

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