UBUNTU: SAUCE: mm: fix memory hotplug in ZONE_HIGHMEM

BugLink: http://bugs.launchpad.net/bugs/1732463



Prior to f1dd2cd1 "mm, memory_hotplug: do not associate
hotadded memory to zones until online" 32-bit x86 with
CONFIG_HIGHMEM=y would default to ZONE_HIGHMEM for hotplugged
memory. That commit changed this to ZONE_NORMAL and made it
impossible for hotplugged memory to be added to ZONE_HIGHMEM,
resulting in oopses whenever the kernel tries to use hotplugged
memory that should have been placed in ZONE_HIGHMEM.

This has been reported upstream, but as a temporary fix make the
following changes:

 - If CONFIG_HIGHMEM=y, also look in ZONE_HIGHMEM when searching
   for a matching zone for memory being onlined.

 - Allow the arch to specify the default zone to be used if no
   matching zone is found.

 - Change 32-bit x86 to set the default zone to ZONE_HIGHMEM if
   CONFIG_HIGHMEM=y.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>