From: Trond Myklebust <trond.myklebust@fys.uio.no>

The patch fixes some problems with NFS under heavy writeout.

NFS pages can be in a clean but unreclaimable state.  They are unreclaimable
because the server has not yet acked the write - we may need to "redirty"
them if the server crashes.

These are referred to as "unstable" pages.  We need to count them alongside
dirty and writeback pages when making flushing and throttling decisions.
Otherwise the machine can be flooded with these pages and the VM has
problems.