zram: remove UNDER_WB and simplify writeback

We now have only one active post-processing at any time, so we don't have
same race conditions that we had before.  If slot selected for
post-processing gets freed or freed and reallocated it loses its PP_SLOT
flag and there is no way for such a slot to gain PP_SLOT flag again until
current post-processing terminates.

Link: https://lkml.kernel.org/r/20240917021020.883356-8-senozhatsky@chromium.org


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