x86: not overmap more than the end of RAM in init_memory_mapping - 64bit

handle head and tail that are not aligned to big pages (2MB/1GB boundary).

with this patch, on system that support gbpages, change:

  last_map_addr: 1080000000 end: 1078000000

to:

  last_map_addr: 1078000000 end: 1078000000

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>