mm/memory_hotplug: disable the functionality for 32b

Memory hotlug is broken for 32b systems at least since c6f03e29 ("mm,
memory_hotplug: remove zone restrictions") which has considerably reworked
how can be memory associated with movable/kernel zones.  The same is not
really trivial to achieve in 32b where only lowmem is the kernel zone.
While we can tweak this immediate problem around there are likely other
land mines hidden at other places.

It is also quite dubious that there is a real usecase for the memory
hotplug on 32b in the first place.  Low memory is just too small to be
hotplugable (for hot add) and generally unusable for hotremove.  Adding
more memory to highmem is also dubious because it would increase the low
mem or vmalloc space pressure for memmaps.

Restrict the functionality to 64b systems.  This will help future
development to focus on usecases that have real life application.  We can
remove this restriction in future in presence of a real life usecase of
course but until then make it explicit that hotplug on 32b is broken and
requires a non trivial amount of work to fix.

Robin said:
 "32-bit Arm doesn't support memory hotplug, and as far as I'm aware
  there's little likelihood of it ever wanting to. FWIW it looks like
  SuperH is the only pure-32-bit architecture to have hotplug support at
  all"

Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Wei Yang <richardw.yang@linux.intel.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Vamshi K Sthambamkadi <vamshi.k.sthambamkadi@gmail.com>
Link: http://lkml.kernel.org/r/20200218100532.GA4151@dhcp22.suse.cz
Link: https://bugzilla.kernel.org/show_bug.cgi?id=206401


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>