When splitting a value entry, we may need to add the new nodes to the LRU
list and remove the parent node from the LRU list.  The WARN_ON checks
in shadow_lru_isolate() catch this oversight.  This bug was latent
until we stopped splitting folios in shrink_page_list() with commit
820c4e2e ("mm/vmscan: Free non-shmem folios without splitting them").
That allows the creation of large shadow entries, and subsequently when
trying to page in a small page, we will split the large shadow entry
in __filemap_add_folio().

Fixes: 8fc75643 ("XArray: add xas_split")
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>