zram: introduce recompress sysfs knob

zram: introduce recompress sysfs knob

Allow zram to recompress (using secondary compression streams)
pages.

Re-compression algorithms (we support up to 3 at this stage)
are selected via recomp_algorithm:

  echo "algo=zstd priority=1" > /sys/block/zramX/recomp_algorithm

Please read documentation for more details.

We support several recompression modes:

1) IDLE pages recompression is activated by `idle` mode

  echo "type=idle" > /sys/block/zram0/recompress

2) Since there may be many idle pages user-space may pass a size
threshold value (in bytes) and we will recompress pages only
of equal or greater size:

  echo "threshold=888" > /sys/block/zram0/recompress

3) HUGE pages recompression is activated by `huge` mode

  echo "type=huge" > /sys/block/zram0/recompress

4) HUGE_IDLE pages recompression is activated by `huge_idle` mode

  echo "type=huge_idle" > /sys/block/zram0/recompress

[senozhatsky@chromium.org: we should always zero out err variable in recompress loop[
Link: https://lkml.kernel.org/r/20221110143423.3250790-1-senozhatsky@chromium.org
Link: https://lkml.kernel.org/r/20221109115047.2921851-5-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Alexey Romanov <avromanov@sberdevices.ru>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>