Base decision to switch on selection or default of protocol

to match the decision mechanism of [1] and [2].

[1]: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_and_managing_networking/getting-started-with-multipath-tcp_configuring-and-managing-networking
[2]: multipath-tcp/mptcp_net-next#18 (comment)

mptcp_mode.c

@@ -35,7 +35,7 @@ int socket(int domain, int type, int protocol)
 {
     _mptcpmode_load();
 
-    if (((PF_INET == domain) || (PF_INET6 == domain))
+    if (((IPPROTO_TCP == protocol) || (0 == protocol))
         && (SOCK_STREAM == type)) {
 
 #ifdef DEBUG