x86: fix bootmem cross node for 32bit numa, cleanup

Impact: clean up

Simplify the code, reuse some lines.
Remove min_low_pfn reference, it is always 0

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <49AEE2C4.2030602@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>