staging: zsmalloc: access page->private by using page_private macro

Change-Id: Ia09720c7787bc03c69d01874437bdc619cae8c7e
Signed-off-by: Sunghan Suh <sunghan.suh@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: e842b976a88a39b447fc34bd0fcb3c0be0a1d9d9
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>