workqueue: fix a bug when numa mapping is changed v4

This is v4. Thank you for hints/commentes to previous versions.

I think this versions only contains necessary things and not invasive. Tested several patterns of node hotplug and seems to work well.

Changes since v3
- removed changes against get_unbound_pool()
- remvoed codes in cpu offline event.
- added node unregister callback.
clear wq_numa_possible_mask at node offline rather than cpu offline.
- updates per-cpu pool's pool-> node at node_(un)register.
- added more comments.
- almost all codes are under CONFIG_MEMORY_HOTPLUG

 include/linux/memory_hotplug.h |    3 +
 kernel/workqueue.c             |   81 ++++++++++++++++++++++++++++++++++++++++-
 mm/memory_hotplug.c            |    6 ++-
 3 files changed, 88 insertions(+), 2 deletions(-)

Original problem was a memory allocation failure because pool->node points to not-online node. This happens when cpu<->node mapping changes.

Yasuaki Ishimatsu hit a allocation failure bug when the numa mapping between CPU and node is changed. This was the last scene:
SLUB: Unable to allocate memory on node 2 (gfp=0x80d0)
cache: kmalloc-192, object size: 192, buffer size: 192, default order: 1, min order: 0 node 0: slabs: 6172, objs: 259224, free: 245741
node 1: slabs: 3261, objs: 136962, free: 127656