early_res: Enhance check_and_double_early_res

... to make it always try to start from low at first.

This makes it less likely for early_memtest to reserve a bad range, in
particular it puts new early_res in a range that is already tested.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <1265793639-15071-24-git-send-email-yinghai@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>