zram: change scan_slots to return void

zram: change scan_slots to return void

scan_slots_for_writeback() and scan_slots_for_recompress() work in a "best
effort" fashion, if they cannot allocate memory for a new pp-slot
candidate they just return and post-processing selects slots that were
successfully scanned thus far.  scan_slots functions never return errors
and their callers never check the return status, so convert them to return
void.

Link: https://lkml.kernel.org/r/20260317032349.753645-1-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>