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: Thomas Gleixner <[e-mail]>
Cc: Richard Weinberger <[e-mail]>

kernel/irq/manage.c

···
 	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);