NFSD: allow OP_DESTROY_CLIENTID to be only op in COMPOUND

   DESTROY_CLIENTID MAY be preceded with a SEQUENCE operation as long as
   the client ID derived from the session ID of SEQUENCE is not the same
   as the client ID to be destroyed.  If the client IDs are the same,
   then the server MUST return NFS4ERR_CLIENTID_BUSY.

(that's not implemented yet)

   If DESTROY_CLIENTID is not prefixed by SEQUENCE, it MUST be the only
   operation in the COMPOUND request (otherwise, the server MUST return
   NFS4ERR_NOT_ONLY_OP).

This fixes the error return; before, we returned
NFS4ERR_OP_NOT_IN_SESSION; after this patch, we return NFS4ERR_NOTSUPP.

Signed-off-by: Benny Halevy <benny@tonian.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>