rcutorture: Reduce SRCU-N number of CPUs

Both SRCU-P and SRCU-N specify eight CPUs, which results in four
iterations for a parallel run on 32 CPUs.  This commit reduces SRCU-N
to four CPUs (but leaving SRCU-P at eight) to speed up parallel runs,
while maintaining essentially the same test coverage.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

tools/testing/selftests/rcutorture/configs/SRCU-N

 CONFIG_RCU_TRACE=n
 CONFIG_SMP=y
-CONFIG_NR_CPUS=8
+CONFIG_NR_CPUS=4
 CONFIG_HOTPLUG_CPU=y
 CONFIG_PREEMPT_NONE=y
 CONFIG_PREEMPT_VOLUNTARY=n