mm/zpool: zbud/zsmalloc implement zpool

Update zbud and zsmalloc to implement the zpool api.

[Fengguang Wu <fengguang.wu@intel.com>: make functions static]
Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Cc: Seth Jennings <sjennings@variantweb.net>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Weijie Yang <weijie.yang@samsung.com>
mm/zpool: update zswap to use zpool

Change zswap to use the zpool api instead of directly using zbud.
Add a boot-time param to allow selecting which zpool implementation
to use, with zbud as the default.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Cc: Seth Jennings <sjennings@variantweb.net>
Cc: Weijie Yang <weijie.yang@samsung.com>