mm/zsmalloc: remove set_zspage_mapping()

Patch series "mm/zsmalloc: some cleanup for get/set_zspage_mapping()".

The discussion[1] with Sergey shows there are some cleanup works to do
in get/set_zspage_mapping():

- the fullness returned from get_zspage_mapping() is not stable outside
  pool->lock, this usage pattern is confusing, but should be ok in this
  free_zspage path.

- we seldom use the class_idx returned from get_zspage_mapping(), only
  free_zspage path use to get its class.

- set_zspage_mapping() always set the zspage->class, but it's never
  changed after zspage allocated.

[1] https://lore.kernel.org/all/a6c22e30-cf10-4122-91bc-ceb9fb57a5d6@bytedance.com/


This patch (of 3):

We only need to update zspage->fullness when insert_zspage(), since
zspage->class is never changed after allocated.

Link: https://lkml.kernel.org/r/20240220-b4-zsmalloc-cleanup-v1-0-5c5ee4ccdd87@bytedance.com
Link: https://lkml.kernel.org/r/20240220-b4-zsmalloc-cleanup-v1-1-5c5ee4ccdd87@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>