Introduce recomp_algorithm sysfs knob that controls secondary algorithm
selection used for recompression.

We will support up to 3 secondary compression algorithms which are sorted
in order of their priority.  To select an algorithm user has to provide
its name and priority:

  echo "algo=zstd priority=1" > /sys/block/zramX/recomp_algorithm
  echo "algo=deflate priority=2" > /sys/block/zramX/recomp_algorithm

During recompression zram iterates through the list of registered
secondary algorithms in order of their priorities.

We also have a short version for cases when there is only
one secondary compression algorithm:

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

This will register zstd as the secondary algorithm with priority 1.

Link: https://lkml.kernel.org/r/20221109115047.2921851-3-senozhatsky@chromium.orgSigned-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@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>