bootmem: Reimplement __absent_pages_in_range() using for_each_mem_pfn_range()

bootmem: Reimplement __absent_pages_in_range() using for_each_mem_pfn_range()

__absent_pages_in_range() was needlessly complex.  Reimplement it
using for_each_mem_pfn_range().

Also, update zone_absent_pages_in_node() such that it doesn't call
__absent_pages_in_range() with @zone_start_pfn which is larger than
@zone_end_pfn.

Signed-off-by: Tejun Heo <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Cc: Yinghai Lu <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Signed-off-by: H. Peter Anvin <[email protected]>