MFC r367008: TCP Cubic: improve reaction to (and rollback from) RTO

fix compliancy issue of CUBIC RTO handling according to RFC8312 section 4.7
add CUBIC CC_RTO_ERR handling

Submitted by: chengc_netapp.com
Reviewed by: rrs, tuexen, rscheff
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D26808