TCP BBR: simplify expression

rsm cannot be NULL, when calling bbr_update_bbr_info().
So no need to check partially for it. No functional change intended.

Reviewed by: rrs
CID: 1523803
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48293

(cherry picked from commit 4173a3a009a42c47f1ec2d1cbfb99347aaf195da)