zsmalloc: do not scan for allocated objects in empty zspage

Patch series "zsmalloc: small compaction improvements", v2.

A tiny series that can reduce the number of find_alloced_obj() invocations
(which perform a linear scan of sub-page) during compaction.  Inspired by
Alexey Romanov's findings.


This patch (of 3):

zspage migration can terminate as soon as it moves the last allocated
object from the source zspage.  Add a simple helper zspage_empty() that
tests zspage ->inuse on each migration iteration.

Link: https://lkml.kernel.org/r/20230624053120.643409-2-senozhatsky@chromium.org


Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Suggested-by: Alexey Romanov <AVRomanov@sberdevices.ru>
Reviewed-by: Alexey Romanov <avromanov@sberdevices.ru>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>