Impact: clean up

Simplify the code, reuse some lines.
Remove min_low_pfn reference, it is always 0
Signed-off-by: NYinghai Lu <yinghai@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <49AEE2C4.2030602@kernel.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>