rcutorture: Reduce CPUs dedicated to testing Classic SRCU

Given that the plan is to retire Classic SRCU in the near future, this
commit reduces the number of CPUs dedicated to testing Classic SRCU.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

tools/testing/selftests/rcutorture/configs/rcu/SRCU-C

 CONFIG_RCU_TRACE=n
 CONFIG_SMP=y
-CONFIG_NR_CPUS=8
+CONFIG_NR_CPUS=3
 CONFIG_HOTPLUG_CPU=y
 CONFIG_RCU_EXPERT=y
 CONFIG_CLASSIC_SRCU=y