tcp: improve KASSERT in limited retransmit

When doing a limited retransmit, allow up to 2 * MSS - 1 if the
Nagle algorithm has been disabled.

PR: 282605
Approved by: re (cperciva)
Reviewed by: cc, Peter Lei
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D49922

(cherry picked from commit 934caaec3afc43638c2a1da8fbe3b672566db4fe)
(cherry picked from commit 0906658c3409996b26518e67df48c01052ef934c)