pNFS: On error, do not send LAYOUTGET until the LAYOUTRETURN has completed

If there is an I/O error, we should not call LAYOUTGET until the
LAYOUTRETURN that reports the error is complete.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: stable@vger.kernel.org # v4.8+