zsmalloc: skip chain size calculation for pow_of_2 classes

If a class size is power of 2 then it wastes no memory and the best
configuration is 1 physical page per-zspage.

Link: https://lkml.kernel.org/r/20230118005210.2814763-3-senozhatsky@chromium.org


Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>