mm: make shrink_page_list with pages work from multiple zones

Shrink_page_list expects all pages come from a same zone
but it's too limited to use.

This patch removes the dependency so next patch can use
shrink_page_list with pages from multiple zones.

Change-Id: I34469b7f0a79f2b79e30e40033ba8b3e1dd5f2d0
Signed-off-by: Minchan Kim <minchan@kernel.org>
Patch-mainline: linux-mm @ 9 May 2013 16:21:25
[vinmenon@codeaurora.org: trivial merge conflict fixes]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>