pnfs: Fix a problem where we gratuitously start doing I/O through the MDS

commit 58bbeab425c6c5e318f5b6ae31d351331ddfb34b upstream.

If the client has to stop in pnfs_update_layout() to wait for another
layoutget to complete, it currently exits and defaults to I/O through
the MDS if the layoutget was successful.

Fixes: d03360aa ("pNFS: Ensure we return the error if someone kills...")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: stable@vger.kernel.org # v4.20+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>