TCP RACK: simplify condition

It is already known that rsm != NULL, so no need to check for it.

Reviewed by: rrs
CID: 1523815
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48282

(cherry picked from commit 8471791eb6eeedaedd4d70e1076cfd143bf12fca)