genirq/debugfs: Reset domain debugfs_file on removal of the debugfs file

When removing a debugfs file for a given irq domain, we fail to clear the
corresponding field, meaning that the corresponding domain won't be created
again if we need to do so.

It turns out that this is exactly what irq_domain_update_bus_token does
(delete old file, update domain name, recreate file).

This doesn't have any impact other than making debug more difficult, but we
do value ease of debugging... So clear the debugfs_file field.
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20181001100522.180054-2-marc.zyngier@arm.com

kernel/irq/irqdomain.c

@@ -1741,6 +1741,7 @@ static void debugfs_add_domain_dir(struct irq_domain *d)
 static void debugfs_remove_domain_dir(struct irq_domain *d)
 {
 	debugfs_remove(d->debugfs_file);
+	d->debugfs_file = NULL;
 }
 
 void __init irq_domain_debugfs_init(struct dentry *root)