net/sched: act_api: fix error code in tcf_ct_flow_table_fill_tuple_ipv6()

The tcf_ct_flow_table_fill_tuple_ipv6() function is supposed to return
false on failure. It should not return negatives because that means
succes/true.

Fixes: fcb6aa86532c ("act_ct: Support GRE offload")
Signed-off-by: Dan Carpenter <[e-mail]>
Acked-by: Toshiaki Makita <[e-mail]>
Link: https://lore.kernel.org/r/YpYFnbDxFl6tQ3Bn@kili
Signed-off-by: Paolo Abeni <[e-mail]>

net/sched/act_ct.c

···
 		break;
 #endif
 	default:
-		return -1;
+		return false;
 	}
 
 	if (ip6h->hop_limit <= 1)