irqchip/gic-v3-its: Fix get_vlpi_map() breakage with doorbells

When updating an LPI configuration, get_vlpi_map() may be passed a
irq_data structure relative to an ITS domain (the normal case) or one
that is relative to the core GICv3 domain in the case of a GICv4
doorbell.

In the latter case, special care must be take not to dereference
the irq_chip data as an its_dev structure, as that isn't what is
stored there. Instead, check *first* whether the IRQ is forwarded
to a vcpu, and only then try to obtain the vlpi mapping.

Fixes: c1d4d5cd ("irqchip/gic-v3-its: Add its_vlpi_map helpers")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20200122085609.658-1-yuzenghui@huawei.com

drivers/irqchip/irq-gic-v3-its.c

@@ -1170,13 +1170,14 @@ static void its_send_vclear(struct its_device *dev, u32 event_id)
  */
 static struct its_vlpi_map *get_vlpi_map(struct irq_data *d)
 {
+	if (irqd_is_forwarded_to_vcpu(d)) {
 		struct its_device *its_dev = irq_data_get_irq_chip_data(d);
 		u32 event = its_get_event_id(d);
 
-	if (!irqd_is_forwarded_to_vcpu(d))
-		return NULL;
-
 		return dev_event_to_vlpi_map(its_dev, event);
+	}
+
+	return NULL;
 }
 
 static void lpi_write_config(struct irq_data *d, u8 clr, u8 set)