rcu: add tracing for RCU's kthread run states.

Add tracing to help debugging situations when RCU's kthreads are not
running but are supposed to be.

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

diff --cc kernel/rcutree.c
Simple merge

diff --cc kernel/rcutree.h
Simple merge

diff --cc kernel/rcutree_plugin.h
Simple merge

diff --cc kernel/rcutree_trace.c
Simple merge