zram: introduce zcomp_ctx structure

Keep run-time driver data (scratch buffers, etc.) in zcomp_ctx structure. 
This structure is allocated per-CPU because drivers (backends) need to
modify its content during requests execution.

We will split mutable and immutable driver data, this is a preparation
path.

Link: https://lkml.kernel.org/r/20240902105656.1383858-19-senozhatsky@chromium.org


Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>