zram: reject unrecognized type= values in recompress_store()

recompress_store() parses the type= parameter with three if statements
checking for "idle", "huge", and "huge_idle". An unrecognized value
silently falls through with mode left at 0, causing the recompression pass
to run with no slot filter — processing all slots instead of the
intended subset.

Add a !mode check after the type parsing block to return -EINVAL for
unrecognized values, consistent with the function's other parameter
validation.

Link: https://lore.kernel.org/20260407153027.42425-1-astellman@stellman-greene.com
Signed-off-by: Andrew Stellman <astellman@stellman-greene.com>
Suggested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>