zram: drop pp_in_progress

pp_in_progress makes sure that only one post-processing (writeback or
recomrpession) is active at any given time.  Functionality wise it,
basically, shadows zram init_lock, when init_lock is acquired in writer
mode.

Switch recompress_store() and writeback_store() to take zram init_lock in
writer mode, like all store() sysfs handlers should do, so that we can
drop pp_in_progress.  Recompression and writeback can be somewhat slow, so
holding init_lock in writer mode can block zram attrs reads, but in
reality the only zram attrs reads that take place are mm_stat reads, and
usually it's the same process that reads mm_stat and does recompression or
writeback.

Link: https://lkml.kernel.org/r/20251216071342.687993-1-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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