Advisory [CVE-2026-64031] fix managed cache race for unaligned extents

In the Linux kernel, the following vulnerability has been resolved: erofs: fix managed cache race for unaligned extents After unaligned compressed extents were introduced, the following race could occur: [Thread 1] [Thread 2] (z_erofs_fill_bio_vec)... filemap_add_folio (1) (z_erofs_bind_cache).... folio_attach_private (2) filemap_add_folio (3) again Since (1) is executed but (2) hasn't been executed yet, it's possible that another thread finds the same managed folio in z_erofs_bind_cache() for a different pcluster and calls filemap_add_folio() again since folio->private is still Z_EROFS_PREALLOCATED_FOLIO.

Fix this by explicitly clearing folio->private before making the folio visible in the managed cache so that another pcluster can simply wait on the locked managed folio as what we did for other shared cases [1].

This only impacts unaligned data compression (`-E48bit` with zstd, for example). [1] Commit 9e2f9d34dd12 ("erofs: handle overlapped pclusters out of crafted images properly") was originally introduced to handle crafted overlapped extents, but it addresses unaligned extents as well.

A race condition exists when processing unaligned compressed extents, which can cause a managed memory page (folio) to be incorrectly added to the page cache multiple times.