NFS: Try to commit unstable writes in nfs_release_page()

If someone calls nfs_release_page(), we presumably already know that the
page is clean, however it may be holding an unstable write.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@kernel.org
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>