xenbus: check xen_domain in xenbus_probe_initcall

The same way we already do in xenbus_init.
Fixes the following warning:

[  352.175563] Trying to free already-free IRQ 0
[  352.177355] WARNING: CPU: 1 PID: 88 at kernel/irq/manage.c:1893 free_irq+0xbf/0x350
[...]
[  352.213951] Call Trace:
[  352.214390]  <TASK>
[  352.214717]  ? __warn+0x81/0x170
[  352.215436]  ? free_irq+0xbf/0x350
[  352.215906]  ? report_bug+0x10b/0x200
[  352.216408]  ? prb_read_valid+0x17/0x20
[  352.216926]  ? handle_bug+0x44/0x80
[  352.217409]  ? exc_invalid_op+0x13/0x60
[  352.217932]  ? asm_exc_invalid_op+0x16/0x20
[  352.218497]  ? free_irq+0xbf/0x350
[  352.218979]  ? __pfx_xenbus_probe_thread+0x10/0x10
[  352.219600]  xenbus_probe+0x7a/0x80
[  352.221030]  xenbus_probe_thread+0x76/0xc0

Fixes: 5b3353949e ("xen: add support for initializing xenstore later as HVM domain")
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

Link: https://lore.kernel.org/r/alpine.DEB.2.22.394.2307211609140.3118466@ubuntu-linux-20-04-desktop
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/xenbus/xenbus_probe.c

@@ -811,6 +811,9 @@ static int xenbus_probe_thread(void *unused)
 
 static int __init xenbus_probe_initcall(void)
 {
+	if (!xen_domain())
+		return -ENODEV;
+
 	/*
 	 * Probe XenBus here in the XS_PV case, and also XS_HVM unless we
 	 * need to wait for the platform PCI device to come up or