when 1/3 user/kernel split is used, and less memory is installed, or if
we have a big hole below 4g, max_low_pfn is still using 3g-128m

try to go down from max_low_pfn until we get it. otherwise will panic.

need to make 32-bit code to use register_e820_active_regions ... later.
Signed-off-by: NYinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>