zram: add recompression algorithm sysfs knob

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/[email protected]
Signed-off-by: Sergey Senozhatsky <[email protected]>
Acked-by: Minchan Kim <[email protected]>
Cc: Alexey Romanov <[email protected]>
Cc: Nhat Pham <[email protected]>
Cc: Nitin Gupta <[email protected]>
Cc: Suleiman Souhlal <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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