genirq: Allow NULL affinity for setup_percpu_irq()

setup_percpu_irq() was forgotten when the percpu_devid infrastructure was
updated to deal with CPU affinities.

In order to keep ignoring users of this legacy API, provide sensible
defaults by setting the affinity to cpu_online_mask if none was provided by
the caller.

Fixes: bdf4e2a ("genirq: Allow per-cpu interrupt sharing for non-overlapping affinities")
Reported-by: Daniel Thompson <danielt@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251205091814.3944205-1-maz@kernel.org
Closes: https://lore.kernel.org/r/aTFozefMQRg7lYxh@aspen.lan

Author: Marc Zyngier

kernel/irq/manage.c

@@ -2470,6 +2470,9 @@ int setup_percpu_irq(unsigned int irq, struct irqaction *act)
 	if (retval < 0)
 		return retval;
 
+	if (!act->affinity)
+		act->affinity = cpu_online_mask;
+
 	retval = __setup_irq(irq, desc, act);
 
 	if (retval)