Allow a TCP function block (tfb) to become the default stack only if tfb->tfb_flags has the TCP_FUNC_DEFAULT_OK flags set. This allows a TCP function block, that is not suitable as a default function block to ensure that it is not set as the default via sysctl. In this case sysctl would return EINVAL.
One could do the same thing using a flag TCP_FUNC_DEFAULT_NOT_OK. Then this change would be smaller, but doing it the way described above seems safer. But if you prefer this approach, we can use it. I'm open to that.