TCP RACK: simplify condition

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

Reviewed by:		rrs
CID:			1523815
MFC after:		1 week
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D48282

sys/netinet/tcp_stacks/rack.c

@@ -18877,7 +18877,7 @@ rack_fast_rsm_output(struct tcpcb *tp, struct tcp_rack *rack, struct rack_sendma
 		log.u_bbr.pkts_out = tp->t_maxseg;
 		log.u_bbr.timeStamp = cts;
 		log.u_bbr.inflight = ctf_flight_size(rack->rc_tp, rack->r_ctl.rc_sacked);
-		if (rsm && (rsm->r_rtr_cnt > 0)) {
+		if (rsm->r_rtr_cnt > 0) {
 			/*
 			 * When we have a retransmit we want to log the
 			 * burst at send and flight at send from before.