TCP BBR: simplify expression

There is no need to check partially for bbr->r_ctl.crte being NULL,
since this can't be true in this path.
No functional change intended.

Reviewed by: rrs
CID: 1523810
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48312

(cherry picked from commit 305c40dc552f9c150eacce95c181798031368cd9)