xen/x86: Initialize per_cpu(xen_vcpu, 0) a little earlier

xen_cpuhp_setup() calls mutex_lock() which, when CONFIG_DEBUG_MUTEXES
is defined, ends up calling xen_save_fl(). That routine expects
per_cpu(xen_vcpu, 0) to be already initialized.
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/x86/xen/enlighten.c

@@ -1644,7 +1644,6 @@ asmlinkage __visible void __init xen_start_kernel(void)
 	xen_initial_gdt = &per_cpu(gdt_page, 0);
 
 	xen_smp_init();
-	WARN_ON(xen_cpuhp_setup());
 
 #ifdef CONFIG_ACPI_NUMA
 	/*
@@ -1658,6 +1657,8 @@ asmlinkage __visible void __init xen_start_kernel(void)
 	   possible map and a non-dummy shared_info. */
 	per_cpu(xen_vcpu, 0) = &HYPERVISOR_shared_info->vcpu_info[0];
 
+	WARN_ON(xen_cpuhp_setup());
+
 	local_irq_disable();
 	early_boot_irqs_disabled = true;