tcp: allow new reno functions to be called from other CC modules

Some new reno functions use the internal data, but are also called
from functions of other CC modules. Ensure that in this case, the
internal data is not accessed.

Reported by: syzbot+1d219ea351caa5109d4b@syzkaller.appspotmail.com
Reported by: syzbot+b08144f8cad9c67258c5@syzkaller.appspotmail.com
Reviewed by: rrs
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D32649