rcu: Move irq-disabled stall-warning checking to tree_stall.h

The rcu_iw_handler() function's sole purpose in life is to indicate
whether a stalled CPU had interrupts disabled, so it belongs in
kernel/rcu/tree_stall.h.  This commit therefore makes that move,
clarifying its header comment while in the area.

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

kernel/rcu/tree.h

@@ -443,4 +443,5 @@ static void rcu_dynticks_task_exit(void);
 
 /* Forward declarations for tree_stall.h */
 static void record_gp_stall_check_time(void);
+static void rcu_iw_handler(struct irq_work *iwp);
 static void check_cpu_stall(struct rcu_data *rdp);