mm/zsmalloc: remove unused zspage->isolated

mm/zsmalloc: remove unused zspage->isolated

The zspage->isolated is not used anywhere, we don't need to maintain it,
which needs to hold the heavy pool lock to update it, so just remove it.

Link: https://lkml.kernel.org/r/20240219-b4-szmalloc-migrate-v1-3-34cd49c6545b@bytedance.com
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>