ACPI: debug: don't allow debugging when ACPI is disabled

If ACPI is disabled then loading the acpi_dbg module will result in the
following splat when lock debugging is enabled.

  DEBUG_LOCKS_WARN_ON(lock->magic != lock)
  WARNING: CPU: 0 PID: 1 at kernel/locking/mutex.c:938 __mutex_lock+0xa10/0x1290
  Kernel panic - not syncing: panic_on_warn set ...
  CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.9.0-rc8+ #103
  Hardware name: linux,dummy-virt (DT)
  Call trace:
   dump_backtrace+0x0/0x4d8
   show_stack+0x34/0x48
   dump_stack+0x174/0x1f8
   panic+0x360/0x7a0
   __warn+0x244/0x2ec
   report_bug+0x240/0x398
   bug_handler+0x50/0xc0
   call_break_hook+0x160/0x1d8
   brk_handler+0x30/0xc0
   do_debug_exception+0x184/0x340
   el1_dbg+0x48/0xb0
   el1_sync_handler+0x170/0x1c8
   el1_sync+0x80/0x100
   __mutex_lock+0xa10/0x1290
   mutex_lock_nested+0x6c/0xc0
   acpi_register_debugger+0x40/0x88
   acpi_aml_init+0xc4/0x114
   do_one_initcall+0x24c/0xb10
   kernel_init_freeable+0x690/0x728
   kernel_init+0x20/0x1e8
   ret_from_fork+0x10/0x18

This is because acpi_debugger.lock has not been initialized as
acpi_debugger_init() is not called when ACPI is disabled.  Fail module
loading to avoid this and any subsequent problems that might arise by
trying to debug AML when ACPI is disabled.

Fixes: 8cfb0cdf07 ("ACPI / debugger: Add IO interface to access debugger functionalities")
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Jamie Iles <jamie@nuviainc.com>
Cc: 4.10+ <stable@vger.kernel.org> # 4.10+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpi_dbg.c

@@ -749,6 +749,9 @@ static int __init acpi_aml_init(void)
 {
 	int ret;
 
+	if (acpi_disabled)
+		return -ENODEV;
+
 	/* Initialize AML IO interface */
 	mutex_init(&acpi_aml_io.lock);
 	init_waitqueue_head(&acpi_aml_io.wait);