genirq/irqdomain: Check pointer in irq_domain_alloc_irqs_hierarchy()

irq_domain_alloc_irqs_hierarchy() has 3 call sites in the compilation unit
but only one of them checks for the pointer which is being dereferenced
inside the called function. Move the check into the function. This allows
for catching the error instead of the following crash:

Unable to handle kernel NULL pointer dereference at virtual address 00000000
PC is at 0x0
LR is at gpiochip_hierarchy_irq_domain_alloc+0x11f/0x140
...
[<c06c23ff>] (gpiochip_hierarchy_irq_domain_alloc)
[<c0462a89>] (__irq_domain_alloc_irqs)
[<c0462dad>] (irq_create_fwspec_mapping)
[<c06c2251>] (gpiochip_to_irq)
[<c06c1c9b>] (gpiod_to_irq)
[<bf973073>] (gpio_irqs_init [gpio_irqs])
[<bf974048>] (gpio_irqs_exit+0xecc/0xe84 [gpio_irqs])
Code: bad PC value

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200306174720.82604-1-alexander.sverdlin@nokia.com

kernel/irq/irqdomain.c

@@ -1310,6 +1310,11 @@ int irq_domain_alloc_irqs_hierarchy(struct irq_domain *domain,
 				    unsigned int irq_base,
 				    unsigned int nr_irqs, void *arg)
 {
+	if (!domain->ops->alloc) {
+		pr_debug("domain->ops->alloc() is NULL\n");
+		return -ENOSYS;
+	}
+
 	return domain->ops->alloc(domain, irq_base, nr_irqs, arg);
 }
 
@@ -1347,11 +1352,6 @@ int __irq_domain_alloc_irqs(struct irq_domain *domain, int irq_base,
 			return -EINVAL;
 	}
 
-	if (!domain->ops->alloc) {
-		pr_debug("domain->ops->alloc() is NULL\n");
-		return -ENOSYS;
-	}
-
 	if (realloc && irq_base >= 0) {
 		virq = irq_base;
 	} else {