x86/mm: Add missing comments for initial kernel direct mapping

Two sets of comments were lost during patch-series shuffling:

  - comments for init_range_memory_mapping()

  - comments in init_mem_mapping that is helpful for reminding people
    that the pagetable is setup top-down

The comments were written by Yinghai in his patch in:

   https://lkml.org/lkml/2012/11/28/620



This patch reintroduces them.

Originally-From: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/518BC776.7010506@gmail.com


[ Tidied it all up a bit. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>