drivers/block/zram/zram_drv.c: do not keep dangling zcomp pointer after zram reset

drivers/block/zram/zram_drv.c: do not keep dangling zcomp pointer after zram reset

We do all reset operations under write lock, so we don't need to save
->disksize and ->comp to stack variables.  Another thing is that ->comp is
freed during zram reset, but comp pointer is not NULL-ed, so zram keeps
the freed pointer value.

Link: https://lkml.kernel.org/r/20220824035100.971816-1-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>