ARM: Fix broken highmem support

Currently, highmem is selectable, and you can request an increased
vmalloc area.  However, none of this has any effect on the memory
layout since a patch in the highmem series was accidentally dropped.
Moreover, even if you did want highmem, all memory would still be
registered as lowmem, possibly resulting in overflow of the available
virtual mapping space.

The highmem boundary is determined by the highest allowed beginning
of the vmalloc area, which depends on its configurable minimum size
(see commit 60296c71 for details on
this).

We should create mappings and initialize bootmem only for low memory,
while the zone allocator must still be told about highmem.

Currently, memory nodes which are completely located in high memory
are not supported.  This is not a huge limitation since systems
relying on highmem support are unlikely to have discontiguous memory
with large holes.

[ A similar patch was meant to be merged before commit 5f0fbf9e


  and be available  in Linux v2.6.30, however some git rebase screw-up
  of mine dropped the first commit of the series, and that goofage
  escaped testing somehow as well. -- Nico ]

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Reviewed-by: Nicolas Pitre <nico@marvell.com>