x86, mm: Split out split_mem_range from init_memory_mapping

So make init_memory_mapping smaller and readable.

-v2: use 0 instead of nr_range as input parameter found by Yasuaki Ishimatsu.

Suggested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1353123563-3103-3-git-send-email-yinghai@kernel.org
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>