CVE-2026-80729: mm/huge_memory: initialise workingset state before folio split

In the Linux kernel, the following vulnerability has been resolved: mm/huge_memory: initialise workingset state before folio split xas_try_split() adds __GFP_ACCOUNT for page-cache xa_nodes, but __folio_split() leaves the xa_state's xa_lru unset. That lets a live, memcg-charged xa_node exist without being linked into the mapping's shadow_nodes list_lru; when reclaim later walks the list_lru it trips VM_WARN_ON(!css_is_dying()). Use mapping_set_update() to install both the workingset update callback and the shadow_nodes list_lru on the xa_state.