x86: make e820_update_range() handle small range update

Impact: enhance e820 code to handle more cases

Try to handle new range which could be covered by one entry.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: jbeulich@novell.com
LKML-Reference: <49B9F0C1.10402@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>