tcp: mark TCP stacks which can serve as a default stack

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.

Reviewed by: gallatin, Peter Lei
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D45419