genirq: Warn if enable_irq is called before irq is set up

The recent changes in the genirq core unearthed a bug in arch/um which
called enable_irq() before the interrupt was set up.

Warn and return instead of crashing the machine with a NULL pointer
dereference.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Cc: Richard Weinberger <richard@nod.at>

kernel/irq/manage.c

@@ -324,6 +324,10 @@ void enable_irq(unsigned int irq)
 	if (!desc)
 		return;
 
+	if (WARN(!desc->irq_data.chip || !desc->irq_data.chip->irq_enable,
+	    KERN_ERR "enable_irq before setup/request_irq: irq %u\n", irq))
+		return;
+
 	chip_bus_lock(desc);
 	raw_spin_lock_irqsave(&desc->lock, flags);
 	__enable_irq(desc, irq, false);