nfsd4: clarify that renewing expired client is a bug

This can't happen:
	- cl_time is zeroed only by unhash_client_locked, which is only
	  ever called under both the state lock and the client lock.
	- every caller of renew_client() should have looked up a
	  (non-expired) client and then called renew_client() all
	  without dropping the state lock.
	- the only other caller of renew_client_locked() is
	  release_session_client(), which first checks under the
	  client_lock that the cl_time is nonzero.

So make it clear that this is a bug, not something we handle.  I can't
quite bring myself to make this a BUG(), though, as there are a lot of
renew_client() callers, and returning here is probably safer than a
BUG().

We'll consider making it a BUG() after some more cleanup.
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -1022,7 +1022,8 @@ static inline void
 renew_client_locked(struct nfs4_client *clp)
 {
 	if (is_client_expired(clp)) {
-		dprintk("%s: client (clientid %08x/%08x) already expired\n",
+		WARN_ON(1);
+		printk("%s: client (clientid %08x/%08x) already expired\n",
 			__func__,
 			clp->cl_clientid.cl_boot,
 			clp->cl_clientid.cl_id);