rcutorture: Right-size TREE10 CPU consumption

The number of CPUs is tuned to allow "4*CFLIST TREE10" on a large system,
up from "3*CFLIST TREE10" previously.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/rcutorture/configs/rcu/TREE10

 CONFIG_SMP=y
-CONFIG_NR_CPUS=100
+CONFIG_NR_CPUS=56
 CONFIG_PREEMPT_NONE=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n