nfsd: nfsd4_setclientid_confirm mistakenly expires confirmed client.

From RFC 7530 Section 16.34.5:

o  The server has not recorded an unconfirmed { v, x, c, *, * } and
   has recorded a confirmed { v, x, c, *, s }.  If the principals of
   the record and of SETCLIENTID_CONFIRM do not match, the server
   returns NFS4ERR_CLID_INUSE without removing any relevant leased
   client state, and without changing recorded callback and
   callback_ident values for client { x }.

The current code intends to do what the spec describes above but
it forgot to set 'old' to NULL resulting to the confirmed client
to be expired.

Fixes: 2b634821

 ("nfsd: fix clid_inuse on mount with security change")
Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Bruce Fields <bfields@fieldses.org>