virt: acrn: Make remove_cpu sysfs invisible with !CONFIG_HOTPLUG_CPU

Without cpu hotplug support, vCPU cannot be removed from a Service VM.
Don't expose remove_cpu sysfs when CONFIG_HOTPLUG_CPU disabled.

Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Qais Yousef <qais.yousef@arm.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Shuo Liu <shuo.a.liu@intel.com>
Link: https://lore.kernel.org/r/20210221134339.57851-2-shuo.a.liu@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Shuo Liu

drivers/virt/acrn/hsm.c

@@ -404,13 +404,22 @@ static ssize_t remove_cpu_store(struct device *dev,
 }
 static DEVICE_ATTR_WO(remove_cpu);
 
+static umode_t acrn_attr_visible(struct kobject *kobj, struct attribute *a, int n)
+{
+       if (a == &dev_attr_remove_cpu.attr)
+               return IS_ENABLED(CONFIG_HOTPLUG_CPU) ? a->mode : 0;
+
+       return a->mode;
+}
+
 static struct attribute *acrn_attrs[] = {
 	&dev_attr_remove_cpu.attr,
 	NULL
 };
 
 static struct attribute_group acrn_attr_group = {
 	.attrs = acrn_attrs,
+	.is_visible = acrn_attr_visible,
 };
 
 static const struct attribute_group *acrn_attr_groups[] = {