zram: introduce zcomp_ctx structure

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/20240712051850.484318-19-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>