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>