Allow disable to fast_fallback of TCPSocket.new (ruby#12210)

with `Socket.tcp_fast_fallback=`
The functions that `Socket.tcp` had are now also available in `TCPSocket.new`.

Author: shioimm

ext/socket/tcpsocket.c

@@ -60,7 +60,7 @@ tcp_init(int argc, VALUE *argv, VALUE sock)
     VALUE kwargs[4];
     VALUE resolv_timeout = Qnil;
     VALUE connect_timeout = Qnil;
-    VALUE fast_fallback = Qtrue;
+    VALUE fast_fallback = Qnil;
     VALUE test_mode_settings = Qnil;
 
     if (!keyword_ids[0]) {
@@ -81,6 +81,13 @@ tcp_init(int argc, VALUE *argv, VALUE sock)
         if (kwargs[3] != Qundef) { test_mode_settings = kwargs[3]; }
     }
 
+    if (fast_fallback == Qnil) {
+        VALUE socket_class = rb_const_get(rb_cObject, rb_intern("Socket"));
+        ID var_id = rb_intern("@tcp_fast_fallback");
+        fast_fallback = rb_ivar_get(socket_class, var_id);
+        if (fast_fallback == Qnil) fast_fallback = Qtrue;
+    }
+
     return rsock_init_inetsock(sock, remote_host, remote_serv,
                                local_host, local_serv, INET_CLIENT,
                                resolv_timeout, connect_timeout, fast_fallback,