x86: fix e820_update_range()

Impact: fix left range size on head

| commit 5c0e6f03
|    x86: fix code paths used by update_mptable
|    Impact: fix crashes under Xen due to unrobust e820 code

fixes one e820 bug, but introduces another bug.

Need to update size for left range at first in case it is header.

also add __e820_add_region take more parameter.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: jbeulich@novell.com
LKML-Reference: <49B9E286.502@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>