zram: do not open-code comp priority 0

A cosmetic change: do not open-code compression priority 0, use
ZRAM_PRIMARY_COMP instead.

Link: https://lkml.kernel.org/r/20241009042908.750260-1-senozhatsky@chromium.org


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