staging: zcache: fix ppc64 and other arches where PAGE_SIZE!=4K

Replace raw constant 12 with PAGE_SHIFT to fix non-x86 arches and
provoke build failure if PAGE_SHIFT is too big

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>