MFC r361347: With RFC3168 ECN, CWR SHOULD only be sent with new data

Overly conservative data receivers may ignore the CWR flag on other
packets, and keep ECE latched. This can result in continous reduction
of the congestion window, and very poor performance when ECN is
enabled.

PR: 243590
Reviewed by: rgrimes (mentor), rrs
Approved by: rgrimes (mentor, blanket)
MFC after: 3 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D23364