Set the slotid and ND_HASSLOTID flag for NFSv4.1 sequenced operations.

Most NFSv4.1 compound RPCs start with a Sequence operation. For these
cases, save the slotid and note that it is saved by setting ND_HASSLOTID.
This is used by r335568 to free up the session slot and disable it.

MFC after: 2 weeks