mptcp: unbreak JSON endpoint list

the following command:

 # ip -j mptcp endpoint show

prints a JSON array that misses the terminating bracket. Fix this calling
delete_json_obj() to balance the call to new_json_obj().

Fixes: 7e0767cd86 ("add support for mptcp netlink interface")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Andrea Claudi <aclaudi@redhat.com>

ip/ipmptcp.c

@@ -278,7 +278,7 @@ static int mptcp_addr_dump(void)
 		return -2;
 	}
 
-	close_json_object();
+	delete_json_obj();
 	fflush(stdout);
 	return 0;
 }