BUG/MINOR: quic: reappend rxbuf buffer on fake dgram alloc error

A thread must always reappend the rxbuf instance after finishing
datagram reception treatment. This was not the case on one error code
path : when fake datagram allocation fails on datagram requeing.

This issue was introduced with the following patch :
  commit b34d353968
  BUG/MEDIUM: quic: consume contig space on requeue datagram
As such, it must be backported in every version with the above commit.

src/quic_sock.c

@@ -749,6 +749,7 @@ int qc_rcv_buf(struct quic_conn *qc)
 				struct quic_dgram *fake_dgram = pool_alloc(pool_head_quic_dgram);
 				if (!fake_dgram) {
 					/* TODO count lost datagrams */
+					MT_LIST_APPEND(&l->rx.rxbuf_list, &rxbuf->rxbuf_el);
 					continue;
 				}