UPSTREAM: mm: multi-gen LRU: improve lru_gen_exit_memcg()

Add warnings and poison ->next.

Link: https://lkml.kernel.org/r/20230118001827.1040870-6-talumbau@google.com


Change-Id: I53de9e04c1ae941e122b33cd45d2bbb5f34aae0c
Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 37cc9997)
Bug: 274865848
Signed-off-by: T.J. Mercier <tjmercier@google.com>