kernel: Don't allow IRQ affinity masks to have more than one CPU

Even with an affinity mask that has multiple CPUs set, IRQs always run
on the first CPU in their affinity mask. Drivers that register an IRQ
affinity notifier (such as pm_qos) will therefore have an incorrect
assumption of where an IRQ is affined.

Fix the IRQ affinity mask deception by forcing it to only contain one
set CPU.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>

kernel/irq/manage.c

@@ -267,6 +267,8 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask,
 		else
 			prog_mask = &tmp_mask;
 	} else {
+		/* IRQs only run on the first CPU in the affinity mask */
+		mask = cpumask_of(cpumask_first(mask));
 		prog_mask = mask;
 	}