MFC r343920:

Ensure that when using the TCP CDG congestion control and setting the
sysctl variable net.inet.tcp.cc.cdg.smoothing_factor to 0, the smoothing
is disabled. Without this patch, a division by zero orrurs.

PR: 193762
Reviewed by: lstewart@, rrs@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D19071