kernel/cpuset.c: use register_hotmemory_notifier()

Use the new interface, remove one ifdef.  No code size changes.

We could/should have been using __meminit/__meminitdata here but there's
now no point in doing that because all this code is elided at compile time.

Cc: Li Zefan <lizefan@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>