zram: remove writestall zram_stats member

zram: remove writestall zram_stats member

There is no zsmalloc handle allocation slow path now and writestall is not
possible any longer.  Remove it from zram_stats.

Link: https://lkml.kernel.org/r/20250221222958.2225035-7-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>