Give tmpfs its own shmem_vm_writeback (and empty shmem_writepages):
going through the default mpage_writepages is very wrong for tmpfs,
since that may write nearby pages while still mapped into mms, but
"writing" converts pages from tmpfs file identity to swap backing
identity: doing so while mapped breaks assumptions throughout e.g.  the
shared file is liable to disintegrate into private instances.