netfilter: nf_tables: replace WARN_ON by WARN_ON_ONCE for unknown verdicts

Bug might trigger warning for each packet, call WARN_ON_ONCE instead.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_core.c

@@ -260,7 +260,7 @@ nft_do_chain(struct nft_pktinfo *pkt, void *priv)
 	case NFT_RETURN:
 		break;
 	default:
-		WARN_ON(1);
+		WARN_ON_ONCE(1);
 	}
 
 	if (stackptr > 0) {