zram: remove chained recompression

zram: remove chained recompression

Chained recompression has unpredictable behavior and is not useful in
practice.

First, systems usually configure just one alternative recompression
algorithm, which has slower compression/decompression but better
compression ratio.  A single alternative algorithm doesn't need chaining.

Second, even with multiple recompression algorithms, chained recompression
is suboptimal.  If a lower priority algorithm succeeds, the page is never
attempted with a higher priority algorithm, leading to worse memory
savings.  If a lower priority algorithm fails, the page is still attempted
with a higher priority algorithm, wasting resources on the failed lower
priority attempt.

In either case, the system would be better off targeting a specific
priority directly.

Chained recompression also significantly complicates the code.  Remove it.

Link: https://lkml.kernel.org/r/20260311084312.1766036-6-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: gao xu <gaoxu2@honor.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>