CVE-2026-74600: mm/page_table_check: skip special zero mappings

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

mm/pagetablecheck: skip special zero mappings

pagetablecheckset() and pagetablecheckclear() account mappings based on PageAnon(). Shared zero-page PTEs and huge zero PMDs are special mappings, but pagetablecheck can still account them as file-backed pages.

An unprivileged process can populate enough zero mappings to overflow filemapcount and hit the existing BUGON(). The PTE path can do this with the shared zero page, and the PMD path can do the same with huge zero mappings.

Skip special zero mappings in the user page-table accounting paths. Keep the PTE-side ptespecial() check, and identify huge zero PMDs from the mapped folio instead of pmdspecial(). That covers architectures where pmdspecial() is a no-op without adding hugezeropfn checks to the generic counter helpers.