tcp: Use local CC data only in the correct context

Most CC algos do use local data, and when calling
newreno_cong_signal from there, the latter misinterprets
the data as its own struct, leading to incorrect behavior.

Reported by: chengc_netapp.com
Reviewed By: chengc_netapp.com, tuexen, transport
MFC after: 3 days
Sponsored By: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D30470