MFC r360491: Introduce a lower bound of 2 MSS to TCP Cubic.

Running TCP Cubic together with ECN could end up reducing cwnd down to 1 byte, if the
receiver continously sets the ECE flag, resulting in very poor transmission speeds.

In line with RFC6582 App. B, a lower bound of 2 MSS is introduced, as well as a typecast
to prevent any potential integer overflows during intermediate calculation steps of the
adjusted cwnd.

Reported by: Cheng Cui
Reviewed by: tuexen (mentor)
Approved by: tuexen (mentor)
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D23353