mm: zswap: warn when referencing a dead entry

Put a standard sanity check on zswap_entry_get() for UAF scenario.

Link: https://lkml.kernel.org/r/20240130014208.565554-5-hannes@cmpxchg.org


Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>