x86/topology: Make DEBUG_HOTPLUG_CPU0 pr_info() more descriptive

DEBUG_HOTPLUG_CPU0 debug feature offlines a CPU as early as possible
allowing userspace to boot up without that CPU (so that it is possible
to check for unwanted dependencies towards the offlined CPU). After
doing so it emits a "CPU %u is now offline" pr_info, which is not enough
descriptive of why the CPU was offlined (e.g., one might be running with
a config that triggered some problem, not being aware that CONFIG_DEBUG_
HOTPLUG_CPU0 is set).

Add a bit more of informative text to the pr_info, so that it is
immediately obvious why a CPU has been offlined in early boot stages.

Background:

Got to scratch my head a bit while debugging a WARNING splat related to
the offlining of CPU0. Without being aware yet of this debug option it
wasn't immediately obvious why CPU0 was being offlined by the kernel.
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: fenghua.yu@intel.com
Link: http://lkml.kernel.org/r/20181219151647.15073-1-juri.lelli@redhat.com
[ Merge line-broken line. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/topology.c

@@ -71,7 +71,7 @@ int _debug_hotplug_cpu(int cpu, int action)
 	case 0:
 		ret = cpu_down(cpu);
 		if (!ret) {
-			pr_info("CPU %u is now offline\n", cpu);
+			pr_info("DEBUG_HOTPLUG_CPU0: CPU %u is now offline\n", cpu);
 			dev->offline = true;
 			kobject_uevent(&dev->kobj, KOBJ_OFFLINE);
 		} else