NR_CPUS: Replace NR_CPUS in arch/x86/kernel/genapic_flat_64.c

  * nr_cpu_ids should be used to determine if a percpu area is
    available for a given cpu.

Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>