mm/madvise: skip device-private PMDs in cold and pageout walks

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

mm/madvise: skip device-private PMDs in cold and pageout walks

madvise_cold_or_pageout_pte_range() takes pmd_trans_huge_lock(), whose
pmd_is_huge() check returns true for a device-private PMD. The subsequent
!pmd_present() branch has a VM_BUG_ON() asserting migration is the only
allowed non-present case; a device-private PMD trips it.

Skip device-private PMDs in that non-present branch and continue to
huge_unlock before calling pmd_folio(). Downgrade the check to
VM_WARN_ON_ONCE() so an unexpected PMD softleaf logs a warning rather than
panicking. Drop the thp_migration_supported() guard: it expands to
IS_ENABLED(CONFIG_ARCH_SUPPORTS_PMD_SOFTLEAF), and both
pmd_is_migration_entry() and pmd_is_device_private_entry() already return
false when that config is not selected, so the guard suppresses only the
case where the warning would already be silent.

Potential trigger: an HMM-based GPU driver races with
madvise(MADV_COLD)/MADV_PAGEOUT: pmd_trans_huge(*pmd) reads true, then
migrate_vma_pages() flips the PMD to a device-private entry before the PMD
lock is acquired.