rxrpc: Fix checker warning by not passing always-zero value to ERR_PTR()

Fix the following checker warning:

	net/rxrpc/call_object.c:279 rxrpc_new_client_call()
	warn: passing zero to 'ERR_PTR'

where a value that's always zero is passed to ERR_PTR() so that it can be
passed to a tracepoint in an auxiliary pointer field.

Just pass NULL instead to the tracepoint.

Fixes: a84a46d7 ("rxrpc: Add some additional call tracing")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/call_object.c

@@ -276,7 +276,7 @@ struct rxrpc_call *rxrpc_new_client_call(struct rxrpc_sock *rx,
 		goto error;
 
 	trace_rxrpc_call(call, rxrpc_call_connected, atomic_read(&call->usage),
-			 here, ERR_PTR(ret));
+			 here, NULL);
 
 	spin_lock_bh(&call->conn->params.peer->lock);
 	hlist_add_head(&call->error_link,