x86: print message if nmi_watchdog=2 cannot be enabled

right now if there's no CPU support for nmi_watchdog=2 we'll just
refuse it silently.

print a useful warning.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/kernel/cpu/perfctr-watchdog.c b/arch/x86/kernel/cpu/perfctr-watchdog.c
index 19a3594..b943e10 100644
--- a/arch/x86/kernel/cpu/perfctr-watchdog.c
+++ b/arch/x86/kernel/cpu/perfctr-watchdog.c
@@ -667,8 +667,10 @@
 {
 	if (!wd_ops) {
 		probe_nmi_watchdog();
-		if (!wd_ops)
+		if (!wd_ops) {
+			printk(KERN_INFO "NMI watchdog: CPU not supported\n");
 			return -1;
+		}
 
 		if (!wd_ops->reserve()) {
 			printk(KERN_ERR