rcu: Don't redundantly disable irqs in rcu_irq_{enter,exit}()

This commit replaces a local_irq_save()/local_irq_restore() pair with
a lockdep assertion that interrupts are already disabled.  This should
remove the corresponding overhead from the interrupt entry/exit fastpaths.

This change was inspired by the fact that Iftekhar Ahmed's mutation
testing showed that removing rcu_irq_enter()'s call to local_ird_restore()
had no effect, which might indicate that interrupts were always enabled
anyway.

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

include/linux/rcutiny.h

@@ -181,6 +181,14 @@ static inline void rcu_irq_enter(void)
 {
 }
 
+static inline void rcu_irq_exit_irqson(void)
+{
+}
+
+static inline void rcu_irq_enter_irqson(void)
+{
+}
+
 static inline void rcu_irq_exit(void)
 {
 }