x86, mm: Move zone init from paging_init() on 64-bit

This patch introduces a zone_sizes_init() helper function on
64-bit to make it more similar to 32-bit init.

Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/1320155902-10424-2-git-send-email-penberg@kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>