The current mapping of smp_processor_id() to a CPU processing Tasks-RCU
callbacks makes some assumptions about layout.  This commit therefore
adds a WARN_ON() to check these assumptions.

[ neeraj.upadhyay: Replace nr_cpu_ids with rcu_task_cpu_ids. ]
Signed-off-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>