x86: Allow 1MB of slack between the e820 map and SRAT, not 4GB

It is expected that there might be slight differences between the e820
map and the SRAT table and the intention was that 1MB of slack be allowed.

The calculation comparing e820ram and pxmram assumes the units are bytes,
when they are in fact pages. This means 4GB of slack is being allowed,
not 1MB. This patch makes the correct comparison.

comment is from Mel.

[ Impact: don't accept buggy SRATs that could dump up to 4G of RAM ]

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <4A03E13E.6050107@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>