memcg: keep folio's objcg same as its node

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

memcg: keep folio's objcg same as its node

memcg_reparent_objcgs() has an inherent assumption that a folio's objcg is
the objcg of the folio's node. Folio migration across nodes breaks that
assumption: the new folio simply inherits the old folio's objcg while
living on a different node.

Once the assumption is broken, the reparenting of the folio's objcg and
the reparenting of the folio's LRU list are no longer atomic.
memcg_reparent_objcgs() handles one node per iteration and drops all the
locks in between, so the objcg gets reparented in the iteration for the
objcg's node while the LRU list gets spliced in the iteration for the
folio's node. Any LRU operation on that folio in between resolves its
lruvec through the objcg, and thus takes the lru_lock of the wrong memcg,
not the lru_lock of the list the folio is actually on.

Fix this by selecting the objcg by folio_nid() at charge time, and by
re-deriving it for the destination node in mem_cgroup_migrate() and
mem_cgroup_replace_folio().