zram: recalculate zstd compression params once

zstd compression params depends on level, but are constant for a given
instance of zstd compression backend.  Calculate once (during ctx
creation).

Link: https://lkml.kernel.org/r/20240902105656.1383858-15-senozhatsky@chromium.org


Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>