CVE-2025-22015: mm/migrate: fix shmem xarray update during migration

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

mm/migrate: fix shmem xarray update during migration

A shmem folio can be either in page cache or in swap cache, but not at the same time. Namely, once it is in swap cache, folio->mapping should be NULL, and the folio is no longer in a shmem mapping.

In foliomigratemapping(), to determine the number of xarray entries to update, foliotestswapbacked() is used, but that conflates shmem in page cache case and shmem in swap cache case. It leads to xarray multi-index entry corruption, since it turns a sibling entry to a normal entry during xasstore() (see [1] for a userspace reproduction). Fix it by only using foliotestswapcache() to determine whether xarray is storing swap cache entries or not to choose the right number of xarray entries to update.

[1] https://lore.kernel.org/linux-mm/Z8idPCkaJW1IChjT@casper.infradead.org/

Note: In splithugepage(), foliotestanon() && foliotestswapcache() is used to get swapcache address space, but that ignores the shmem folio in swap cache case. It could lead to NULL pointer dereferencing when a in-swap-cache shmem folio is split at xastore(), since !foliotestanon() is true and folio->mapping is NULL. But fortunately, its caller splithugepagetolisttoorder() bails out early with EBUSY when folio->mapping is NULL. So no need to take care of it here.