TCP RACK, BBR: ensure return value is always ininitialized

Do not return an uninitialized value from ctf_do_queued_segments()
in case no packets are actually processed (all are skipped).

Reviewed by:		rrs
CID:			1523774
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D48217

(cherry picked from commit b5739c8)

sys/netinet/tcp_stacks/rack_bbr_common.c

@@ -520,7 +520,7 @@ ctf_process_inbound_raw(struct tcpcb *tp, struct socket *so, struct mbuf *m, int
 	if (no_vn == 0) {
 		CURVNET_RESTORE();
 	}
-	return(retval);
+	return (0);
 }
 
 int