zsmalloc: rename class stat mutators

zsmalloc: rename class stat mutators

A cosmetic change.

o Rename class_stat_inc() and class_stat_dec() to class_stat_add()
  and class_stat_sub() correspondingly. inc/dec are usually associated
  with +1/-1 modifications, while zsmlloc can modify stats by up
  to ->objs_per_zspage. Use add/sub (follow atomics naming).

o Rename zs_stat_get() to class_stat_read()
  get() is usually associated with ref-counting and is paired with put().
  zs_stat_get() simply reads class stat so rename to reflect it.
  (This also follows atomics naming).

Link: https://lkml.kernel.org/r/20240701031140.3756345-1-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>