netfilter: conntrack: use fallthrough to cleanup

These cases all use the same function. we can simplify the code through
fallthrough.

$ size net/netfilter/nf_conntrack_core.o

        text	   data	    bss	    dec	    hex	filename
before  81601	  81430	    768	 163799	  27fd7	net/netfilter/nf_conntrack_core.o
after   80361	  81430	    768	 162559	  27aff	net/netfilter/nf_conntrack_core.o

Arch: aarch64
Gcc : gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
Reported-by: k2ci <kernel-bot@kylinos.cn>
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_core.c

@@ -329,20 +329,18 @@ nf_ct_get_tuple(const struct sk_buff *skb,
 		return gre_pkt_to_tuple(skb, dataoff, net, tuple);
 #endif
 	case IPPROTO_TCP:
-	case IPPROTO_UDP: /* fallthrough */
-		return nf_ct_get_tuple_ports(skb, dataoff, tuple);
+	case IPPROTO_UDP:
 #ifdef CONFIG_NF_CT_PROTO_UDPLITE
 	case IPPROTO_UDPLITE:
-		return nf_ct_get_tuple_ports(skb, dataoff, tuple);
 #endif
 #ifdef CONFIG_NF_CT_PROTO_SCTP
 	case IPPROTO_SCTP:
-		return nf_ct_get_tuple_ports(skb, dataoff, tuple);
 #endif
 #ifdef CONFIG_NF_CT_PROTO_DCCP
 	case IPPROTO_DCCP:
-		return nf_ct_get_tuple_ports(skb, dataoff, tuple);
 #endif
+		/* fallthrough */
+		return nf_ct_get_tuple_ports(skb, dataoff, tuple);
 	default:
 		break;
 	}