Impact: micro-optimization, memory reduction

On x86_64 flush tlb data is stored in per_cpu variables. This is
unnecessary because only the first NUM_INVALIDATE_TLB_VECTORS entries
are accessed.

This patch aims at making the code less confusing (there's nothing
really "per_cpu") by using a plain array. It also would save some memory
on most distros out there (Ubuntu x86_64 has NR_CPUS=64 by default).

[ Ravikiran G Thirumalai also pointed out that the correct alignment
  is ____cacheline_internodealigned_in_smp, so that there's no
  bouncing on vsmp. ]
Signed-off-by: Frederik Deweerdt <frederik.deweerdt@xprog.eu>
Acked-by: Ravikiran Thirumalai <kiran@scalex86.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>