irqdomain: Use struct_size() helper when allocating irqdomain

Instead of open-coding the size computation of struct irqdomain,
use the struct_size() helper instead.

This is going to be handy as we change the type of the revmap
array.
Signed-off-by: Marc Zyngier <maz@kernel.org>

kernel/irq/irqdomain.c

@@ -150,7 +150,7 @@ struct irq_domain *__irq_domain_add(struct fwnode_handle *fwnode, int size,
 		    (!IS_ENABLED(CONFIG_IRQ_DOMAIN_NOMAP) && direct_max)))
 		return NULL;
 
-	domain = kzalloc_node(sizeof(*domain) + (sizeof(unsigned int) * size),
+	domain = kzalloc_node(struct_size(domain, revmap, size),
 			      GFP_KERNEL, of_node_to_nid(to_of_node(fwnode)));
 	if (!domain)
 		return NULL;