NFS41/flexfiles: zero out DS write wcc

We do not want to update inode attributes with DS values.

Cc: stable@vger.kernel.org # v4.0+
Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>