x86-64, NUMA: Don't call __pa() with invalid address in numa_reset_distance()

Do not call __pa(numa_distance) if it was not allocated before.
Calling with invalid address triggers VIRTUAL_BUG_ON() in
__phys_addr() if CONFIG_DEBUG_VIRTUAL.

Also reported by Ingo.

 http://thread.gmane.org/gmane.linux.kernel/1101306/focus=1101785



- v2: Change to check existing path as tj requested.
- tj: Description update.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Ingo Molnar <mingo@elte.hu>