memcg: move LRU size accounting on reparenting instead of copying it

In the Linux kernel, the following vulnerability has been resolved:

memcg: move LRU size accounting on reparenting instead of copying it

When a memory cgroup is offlined its LRU folios are reparented to the
parent. lruvec_reparent_lru() splices the child's lists into the
parent's and credits the parent with the child's per-zone
lru_zone_size[], but never clears the child's copy, so the size is
copied rather than moved. lru_gen_reparent_memcg() does the same for
MGLRU.

The parent is left correct, credited with exactly the folios it took
over. The stale value sits on the child and nothing will correct it:
folio->memcg_data now resolves to the parent, so every later
update_lru_size() for those folios goes there.

Dying cgroups are not freed immediately and mem_cgroup_iter() still
walks them, so shrink_lruvec() keeps being called on them.
get_scan_count() reads the phantom counter through lruvec_lru_size() and
the scan loop then grinds through nr[] in SWAP_CLUSTER_MAX steps against
an empty list, for as long as the dead cgroup lives. Under MGLRU the
MGLRU scanner runs instead, but count_shadow_nodes() sums all of
NR_LRU_LISTS through lruvec_lru_size() and over-budgets the shadow node
limit just the same.

On one 251 GiB host a sweep of every mz->lru_zone_size[] found 380
counters describing folios on no list at all: 124777314 pages, 476 GiB,
1.89x the machine's RAM, across 57 cgroups. All were on memcgs with
CSS_DYING set and CSS_ONLINE clear, and parent/child pairs reported
byte-identical sizes.

LRU_UNEVICTABLE needs its size moved too. Its list is deliberately not
spliced because lruvec_init() poisons the head - the unevictable LRU is
imaginary and folios are never threaded on it - but the size is kept by
lruvec_add_folio()/lruvec_del_folio() and those folios account to the
parent from here on.

This depends on commit bf4ade7dbd76 ("memcg: keep folio's objcg same as
its node") and must not be backported ahead of it. Without that
invariant a folio's objcg can belong to another node, so a folio already
spliced onto the parent's list can still resolve to the child's lruvec
until the objcg's node is reparented in a later iteration of
memcg_reparent_objcgs(); clearing the child's counter early then lets
lruvec_del_folio() underflow it and trip the WARN_ONCE()/VM_BUG_ON() in
mem_cgroup_update_lru_size().