zsmalloc: Add Kconfig for enabling PTE method

Zsmalloc has two methods 1) copy-based and 2) pte-based to access
allocations that span two pages. You can see history why we supported
two approach from [1].

In summary, copy-based method is 3 times fater in x86 while pte-based
is 6 times faster in ARM.

But it was bad choice that adding hard coding to select architecture
which want to use pte based method. This patch removed it and adds
new Kconfig to select the approach.

This patch is based on next-20130205.

[1] https://lkml.org/lkml/2012/7/11/58

* Changelog from v1
  * Fix CONFIG_PGTABLE_MAPPING in zsmalloc-main.c - Greg

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Seth Jennings <sjenning@linux.vnet.ibm.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: Konrad Rzeszutek Wilk <konrad@darnok.org>
Signed-off-by: Minchan Kim <minchan@kernel.org>

Conflicts:
	drivers/staging/zsmalloc/zsmalloc-main.c