bpf: allow disabling tunnel csum for ipv6

Before the patch, BPF_F_ZERO_CSUM_TX can be used only for ipv4 tunnel.
With introduction of ip6gretap collect_md mode, the flag should be also
supported for ipv6.
Signed-off-by: William Tu <u9012063@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/filter.c

@@ -3026,9 +3026,10 @@ BPF_CALL_4(bpf_skb_set_tunnel_key, struct sk_buff *, skb,
 				  IPV6_FLOWLABEL_MASK;
 	} else {
 		info->key.u.ipv4.dst = cpu_to_be32(from->remote_ipv4);
+	}
+
 	if (flags & BPF_F_ZERO_CSUM_TX)
 		info->key.tun_flags &= ~TUNNEL_CSUM;
-	}
 
 	return 0;
 }