rcu: Dump number of callbacks in stall warning messages

In theory, if a grace period manages to get started despite there being
no callbacks on any of the CPUs, all CPUs could go into dyntick-idle
mode, so that the grace period would never end.  This commit updates
the RCU CPU stall warning messages to detect this condition by summing
up the number of callbacks on all CPUs.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>