mm: Force TLB flush for PFNMAP mappings before unlink_file_vma()

commit b67fbebd upstream.

Some drivers rely on having all VMAs through which a PFN might be
accessible listed in the rmap for correctness.
However, on X86, it was possible for a VMA with stale TLB entries
to not be listed in the rmap.

This was fixed in mainline with
commit b67fbebd ("mmu_gather: Force tlb-flush VM_PFNMAP vmas"),
but that commit relies on preceding refactoring in
commit 18ba064e ("mmu_gather: Let there be one tlb_{start,end}_vma()
implementation") and commit 1e9fdf21

 ("mmu_gather: Remove per arch
tlb_{start,end}_vma()").

This patch provides equivalent protection without needing that
refactoring, by forcing a TLB flush between removing PTEs in
unmap_vmas() and the call to unlink_file_vma() in free_pgtables().

[This is a stable-specific rewrite of the upstream commit!]
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>