x86: max_low_pfn_mapped fix, #3

optimization: try to merge the range with same page size in
init_memory_mapping, to get the best possible linear mappings set up.

thus when GBpages is not there, we could do 2M pages.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>