x86: add the word 'WARNING' in check_nmi_watchdog() output

Our automated test suite looks for keywords like error, fail, warning in
the boot log.  In the case when the nmi watchdog is determined to be
stuck in check_nmi_watchdog(), none of those keywords are displayed.

This patch adds a keyword, "WARNING:", so it makes it easier to notice
when the nmi watchdog isn't working correctly. Also add a proper
KERN_WARNING mark to this printout.

Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>