rcu: Move rcu_cpu_kthread_task to rcu_data structure

rcu: Move rcu_cpu_kthread_task to rcu_data structure

Given that RCU has a perfectly good per-CPU rcu_data structure, most
per-CPU quantities should be stored there.

This commit therefore moves the rcu_cpu_kthread_task per-CPU variable to
the rcu_data structure.  This also makes this variable unconditionally
present, which should be acceptable given the memory reduction due to the
RCU flavor consolidation and also due to simplifications this will enable.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>