zram: add writeback batch size device attr

zram: add writeback batch size device attr

Introduce writeback_batch_size device attribute so that the maximum number
of in-flight writeback bio requests can be configured at run-time
per-device.  This essentially enables batched bio writeback.

Link: https://lkml.kernel.org/r/20251122074029.3948921-3-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@google.com>
Cc: Richard Chang <richardycc@google.com>
Cc: Yuwen Chen <ywen.chen@foxmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>