rcutorture: Fix rcu_torture_cbflood() for callback-free RCU

The rcu_torture_cbflood() function correctly checks for flavors of
RCU that lack analogs to call_rcu() and rcu_barrier(), but in that
case it fails to terminate correctly.  In fact, it terminates so
incorrectly that segfaults can result.  This commit therefore causes
rcu_torture_cbflood() to do the proper wait-for-stop procedure.

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