x86: tight online check in setup_per_cpu_areas

when numa disabled I got this compile warning:

arch/x86/kernel/setup64.c: In function setup_per_cpu_areas:
arch/x86/kernel/setup64.c:147: warning: the address of
                      contig_page_data will always evaluate as true

it seems we missed checking if the node is online before we try to refer
NODE_DATA. Fix it.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>