Add the BBR and RACK stacks to the LINT kernel.

While here, drop the EXTRA_TCP_STACKS option, which serves no purpose and
should never have been added. Instead, build bbr and rack as long as
either or both of INET and INET6 is enabled. There is no risk to anyone
who doesn't load one or both and then twiddle the relevant sysctls.

Differential Revision: https://reviews.freebsd.org/D42088

(cherry picked from commit 3a338c534154164504005beb00a3c6feb03756cc)