powerpc/irq: Fix another case of lazy IRQ state getting out of sync

So we have another case of paca->irq_happened getting out of
sync with the HW irq state. This can happen when a perfmon
interrupt occurs while soft disabled, as it will return to a
soft disabled but hard enabled context while leaving a stale
PACA_IRQ_HARD_DIS flag set.

This patch fixes it, and also adds a test for the condition
of those flags being out of sync in arch_local_irq_restore()
when CONFIG_TRACE_IRQFLAGS is enabled.

This helps catching those gremlins faster (and so far I
can't seem see any anymore, so that's good news).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/irq.c

@@ -229,6 +229,19 @@ notrace void arch_local_irq_restore(unsigned long en)
 	 */
 	if (unlikely(irq_happened != PACA_IRQ_HARD_DIS))
 		__hard_irq_disable();
+#ifdef CONFIG_TRACE_IRQFLAG
+	else {
+		/*
+		 * We should already be hard disabled here. We had bugs
+		 * where that wasn't the case so let's dbl check it and
+		 * warn if we are wrong. Only do that when IRQ tracing
+		 * is enabled as mfmsr() can be costly.
+		 */
+		if (WARN_ON(mfmsr() & MSR_EE))
+			__hard_irq_disable();
+	}
+#endif /* CONFIG_TRACE_IRQFLAG */
+
 	set_soft_enabled(0);
 
 	/*