mm: zpool: implement common zpool api to zbud/zsmalloc

Add zpool api.

zpool provides an interface for memory storage, typically of compressed
memory.  Users can select what backend to use; currently the only
implementations are zbud, a low density implementation with up to
two compressed pages per storage page, and zsmalloc, a higher density
implementation with multiple compressed pages per storage page.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
mm/zpool: zbud/zsmalloc implement zpool

Update zbud and zsmalloc to implement the zpool api.
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>
mm: prevent zbud/zsmalloc from unloading when used

Add try_module_get() to pool creation functions for zbud and zsmalloc,
and module_put() to pool destruction functions, since they now can be
modules used via zpool.  Without usage counting, they could be unloaded
while pool(s) were active, resulting in an oops.