NFS: Fix NFS page_state usage

The introduction of the FLUSH_INVALIDATE argument to nfs_sync_inode_wait()
does not clear the nr_unstable page state counter for pages that are being
released.

Also fix a longstanding similar bug when nfs_commit_list() fails.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>