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>