This adds a new field to zs_pool to store evict handlers for writeback,
analogous to the zbud allocator.

Link: https://lkml.kernel.org/r/20221128191616.1261026-6-nphamcs@gmail.comSigned-off-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>