x86/xen: Allow to retry if cpu_initialize_context() failed.

If memory allocation in cpu_initialize_context() fails then it will
bring up the VCPU and leave with the corresponding CPU bit set in
xen_cpu_initialized_map.

The following (presumably successful) CPU bring up will BUG in
xen_pv_cpu_up() because nothing for that VCPU would be initialized.

Clear the CPU bits, that were set in cpu_initialize_context() in case
the memory allocation fails.

[ bigeasy: Creating a patch from Boris' email. ]
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20220209080214.1439408-2-bigeasy@linutronix.de

arch/x86/xen/smp_pv.c

@@ -260,8 +260,11 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
 		return 0;
 
 	ctxt = kzalloc(sizeof(*ctxt), GFP_KERNEL);
-	if (ctxt == NULL)
+	if (ctxt == NULL) {
+		cpumask_clear_cpu(cpu, xen_cpu_initialized_map);
+		cpumask_clear_cpu(cpu, cpu_callout_mask);
 		return -ENOMEM;
+	}
 
 	gdt = get_cpu_gdt_rw(cpu);