MT#55283 add IPv4 fallback

Try open an IPv4 socket if IPv6 fails with EAFNOSUPPORT in order to
support kernels without IPv6 support.

closes #1784

Change-Id: I2cf4065450a8b19b738c572e7d9ad6f8548ecc04

lib/socket.c

@@ -800,15 +800,19 @@ fail:
 }
 
 int open_v46_socket(socket_t *r, int type) {
-	sockfamily_t *fam = &__socket_families[SF_IP6];
-
-	if (__socket(r, type, fam)) {
-		__C_DBG("open socket fail, fd=%d", r->fd);
-		return -1;
+	int ret = __socket(r, type, &__socket_families[SF_IP6]);
+	if (ret) {
+		if (errno == EAFNOSUPPORT)
+			ret = __socket(r, type, &__socket_families[SF_IP4]);
+		if (ret) {
+			__C_DBG("open socket fail");
+			return -1;
+		}
 	}
+	else
+		ipv6only(r->fd, 0);
 
 	nonblock(r->fd);
-	ipv6only(r->fd, 0);
 
 	return 0;
 }