irqdomain: Check virq for 0 before use in irq_dispose_mapping()

It's a bit hard to read the logic since the virq is used before checking it
for 0. Rearrange the code to make it better to understand.

This, in particular, should clearly answer the question whether the caller
needs to perform this check or not, and there are plenty of places for both
variants, confirming a confusion.

Fun fact that the new code is shorter:

Function old new delta
irq_dispose_mapping 278 271 -7
Total: Before=11625, After=11618, chg -0.06%

when compiled by GCC on Debian for x86_64.

Signed-off-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

kernel/irq/irqdomain.c

···
  */
 void irq_dispose_mapping(unsigned int virq)
 {
-	struct irq_data *irq_data = irq_get_irq_data(virq);
+	struct irq_data *irq_data;
 	struct irq_domain *domain;
 
-	if (!virq || !irq_data)
+	irq_data = virq ? irq_get_irq_data(virq) : NULL;
+	if (!irq_data)
 		return;
 
 	domain = irq_data->domain;