initmem_init() extensively accesses and modifies global data
structures and the parameters aren't even followed depending on which
path is being used.  Drop @start/last_pfn and let it deal with
@max_pfn directly.  This is in preparation for further NUMA init
cleanups.

- v2: x86-32 initmem_init() weren't updated breaking 32bit builds.
  Fixed.  Found by Yinghai.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Shaohui Zheng <shaohui.zheng@intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: H. Peter Anvin <hpa@linux.intel.com>