NFSv4.1: Handle NFS4ERR_DELAY on SEQUENCE correctly

In RFC5661, an NFS4ERR_DELAY error on a SEQUENCE operation has the special
meaning that the server is not finished processing the request. In this
case we want to just retry the request without touching the slot.

Also fix a bug whereby we would fail to update the sequence id if the
server returned any error other than NFS_OK/NFS4ERR_DELAY.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>