tcp: properly unroll SACK transmission on TX error with LRD

PR: 275798
Reported by: ddaniel@nvidia.com
Reviewed by: tuexen, transport
MFC after: 1 week
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43085

(cherry picked from commit e3b9058e5cd0f541da596624a366e14cabcf2e2a)