rcu: Don't do reschedule unless in irq

Condition the set_need_resched() in rcu_irq_exit() on in_irq().  This
should be a no-op, because rcu_irq_exit() should only be called from irq.

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

diff --cc kernel/rcutree.c
Simple merge