rcu: Print number of online CPUs in RCU CPU stall-warning messages

RCU's synchronous grace periods act quite differently when there is
only one online CPU, especially in the no-op case in kernels built with
CONFIG_PREEMPTION=n.  This change in behavior can be important debugging
information, so this commit adds the number of online CPUs to the RCU
CPU stall warning messages.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>