mm/x86: prepare for removing num_physpages and simplify mem_init()

Prepare for removing num_physpages and simplify mem_init().

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andreas Herrmann <andreas.herrmann3@amd.com>
Cc: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Wen Congyang <wency@cn.fujitsu.com>
Cc: Jianguo Wu <wujianguo@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>