x86: set_highmem_pages_init() cleanup, #2

Impact: cleanup

The zones are set up at this stage so there's a highmem zone
available even for the UMA case.

The only difference there is that for machines that have
CONFIG_HIGHMEM enabled but don't have any highmem available,
->zone_start_pfn is zero whereas highstart_pfn is non-zero).

The field is left zeroed because of the !size test in
free_area_init_core() but shouldn't be a problem because
add_highpages_with_active_regions() handles empty ranges just
fine.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Mel Gorman <mel@csn.ul.ie>
LKML-Reference: <1236154567.29024.23.camel@penberg-laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>