zram: introduce custom comp backends API

zram: introduce custom comp backends API

Moving to custom backends implementation gives us ability to
have our own minimalistic and extendable API, and algorithms
tunings becomes possible.

The list of compression backends is empty at this point,
we will add backends in the followup patches.

Link: https://lkml.kernel.org/r/20240712051850.484318-5-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Nick Terrell <terrelln@fb.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>