MINOR: quic: Add a trace to distinguish the datagram from the packets inside

Without such a trace, we do not know when a datagram is sent. Only trace for
the packets inside the datagrams were displayed.

Must be backported to 2.6.
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index 0172c44..bd2153e 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -3203,6 +3203,7 @@
 		tmpbuf.area = (char *)pos;
 		tmpbuf.size = tmpbuf.data = dglen;
 
+		TRACE_DATA("send dgram", QUIC_EV_CONN_SPPKTS, qc);
 		/* If sendto is on error just skip the call to it for the rest
 		 * of the loop but continue to purge the buffer. Data will be
 		 * transmitted when QUIC packets are detected as lost on our