irqchip/gic-v3-its: Tag ITS device as shared if allocating for a proxy device

The ITS already has some notion of "shared" devices. Let's map the
MSI_ALLOC_FLAGS_PROXY_DEVICE flag onto this internal property.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/20201129135208.680293-3-maz@kernel.org

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

@@ -3487,6 +3487,9 @@ static int its_msi_prepare(struct irq_domain *domain, struct device *dev,
 		goto out;
 	}
 
+	if (info->flags & MSI_ALLOC_FLAGS_PROXY_DEVICE)
+		its_dev->shared = true;
+
 	pr_debug("ITT %d entries, %d bits\n", nvec, ilog2(nvec));
 out:
 	mutex_unlock(&its->dev_alloc_lock);