x86, e820, pci: reserve extra free space near end of RAM

The point is to take all RAM resources we have, and
_after_ we've added all the resources we've seen in
the E820 tree, we then _also_ try to add fake reserved
entries for any "round up to X" at the end of the RAM
resources.

[ Impact: improve PCI mem-resource allocation robustness, protect "stolen RAM" ]

Reported-by: Yannick Roehlly <yannick.roehlly@free.fr>
Acked-by: Jesse Barnes <jesse.barnes@intel.com>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: yannick.roehlly@free.fr
LKML-Reference: <4A01A784.2050407@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>