tcp bblog: use correct length

The length of tldl_reason is TCP_LOG_REASON_LEN, not TCP_LOG_ID_LEN.
No functional change intended.
Reported by: Coverity Scan
CID: 1418074
CID: 1418276
Reviewed by: glebius, rscheff
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D44510