zram: preparation for multi-zcomp support

Patch series "zram: Support multiple compression streams", v5.

This series adds support for multiple compression streams.  The main idea
is that different compression algorithms have different characteristics
and zram may benefit when it uses a combination of algorithms: a default
algorithm that is faster but have lower compression rate and a secondary
algorithm that can use higher compression rate at a price of slower
compression/decompression.

There are several use-case for this functionality:

- huge pages re-compression: zstd or deflate can successfully compress
  huge pages (~50% of huge pages on my synthetic ChromeOS tests), IOW
  pages that lzo was not able to compress.

- idle pages re-compression: idle/cold pages sit in the memory and we
  may reduce zsmalloc memory usage if we recompress those idle pages.

Userspace has a number of ways to control the behavior and impact of zram
recompression: what type of pages should be recompressed, size watermarks,
etc.  Please refer to documentation patch.


This patch (of 13):
			
The patch turns compression streams and compressor algorithm name struct
zram members into arrays, so that we can have multiple compression streams
support (in the next patches).

The patch uses a rather explicit API for compressor selection:

- Get primary (default) compression stream
	zcomp_stream_get(zram->comps[ZRAM_PRIMARY_COMP])
- Get secondary compression stream
	zcomp_stream_get(zram->comps[ZRAM_SECONDARY_COMP])

We use similar API for compression streams put().

At this point we always have just one compression stream,
since CONFIG_ZRAM_MULTI_COMP is not yet defined.

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

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