mm: zswap: make the lock critical section obvious in shrink_worker()

mm: zswap: make the lock critical section obvious in shrink_worker()

Move the comments and spin_{lock/unlock}() calls around in shrink_worker()
to make it obvious the lock is protecting the loop updating
zswap_next_shrink.

Link: https://lkml.kernel.org/r/20240803053306.2685541-1-yosryahmed@google.com
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Cc: Takero Funaki <flintglass@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>