pNFS/flexfiles: If the layout is invalid, it must be updated before retrying

If we see that our pNFS READ/WRITE/COMMIT operation failed, but we
also see that our layout segment is no longer valid, then we need to
get a new layout segment before retrying.

Fixes: 90816d1d

 ("NFSv4.1/flexfiles: Don't mark the entire deviceid...")
Cc: stable@vger.kernel.org # v4.2+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>