Don't leak idle function address in NMI backtrace.

Link: http://lkml.kernel.org/r/20171106165648.GA95243@sofiaSigned-off-by: NLiu Changcheng <changcheng.liu@intel.com>
Reviewed-by: NPetr Mladek <pmladek@suse.com>
Reviewed-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>