staging: zsmalloc: add ZS_MAX_PAGES_PER_ZSPAGE

This patch moves where max_zspage_order is declared and
changes its meaning.  "Order" typically implies 2^order
of something; however, it is currently being used as the
"maximum number of single pages in a zspage".  To add clarity,
ZS_MAX_ZSPAGE_ORDER is now used to calculate ZS_MAX_PAGES_PER_ZSPAGE,
which is 2^ZS_MAX_ZSPAGE_ORDER and is the upper bound on the number
of pages in a zspage.

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Acked-by: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>