MINOR: quic: Useless rate sample member initialization

This poor/inefficient code has been revealed by coverity GH issue in #2788 where
some quic_cc_rs struct member initializations were mentionned as overwritten
(after initialization) before being used as follows:

CID 1565821:  Code maintainability issues  (UNUSED_VALUE)
/src/quic_cc_bbr.c: 1373 in bbr_handle_lost_packet()
1367     }
1368
1369     static void bbr_handle_lost_packet(struct bbr *bbr, struct quic_cc_path *p,
1370                                        struct quic_tx_packet *pkt,
1371                                        uint32_t lost)
1372     {
>>>     CID 1565821:  Code maintainability issues  (UNUSED_VALUE)
>>>     Assigning value "0UL" to "rs.tx_in_flight" here, but that stored value is overwritten before it can be used.
1373            struct quic_cc_rs rs = {0};
1374
1375            /* C.delivered = bbr->drs.delivered */
1376            bbr_note_loss(bbr, bbr->drs.delivered);
1377            if (!bbr->bw_probe_samples)
1378                    return; /* not a packet sent while probing bandwidth */

Remove the {0} initializer for <rs> variable. This is safe because the members
initializations of <rs> local variable passed to functions from
bbr_handle_lost_packet() are done. Add a comment to mention this.

src/quic_cc_bbr.c

@@ -1370,13 +1370,16 @@ static void bbr_handle_lost_packet(struct bbr *bbr, struct quic_cc_path *p,
                                    struct quic_tx_packet *pkt,
                                    uint32_t lost)
 {
-	struct quic_cc_rs rs = {0};
+	struct quic_cc_rs rs;
 
 	/* C.delivered = bbr->drs.delivered */
 	bbr_note_loss(bbr, bbr->drs.delivered);
 	if (!bbr->bw_probe_samples)
 		return; /* not a packet sent while probing bandwidth */
 
+	/* Only ->tx_in_fligth, ->lost and ->is_app_limited <rs> member
+	 * initializations are needed.
+	 */
 	rs.tx_in_flight = pkt->rs.tx_in_flight; /* inflight at transmit */
 	BUG_ON(bbr->drs.lost + pkt->len < lost);
 	/* bbr->rst->lost is not yet incremented */