TCP RACK: don't log an uninitialized value

reduce is uninitialized, if the code path for logging is reached via
goto old_method;.

Reviewed by: rrs, Peter Lei
CID: 1557359
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48346

(cherry picked from commit 4c89d59e0cdac4d83fb5841aefae9214545b2273)