lacp: warn transmit failure of lacp pdu

It might be difficult to trace whether LACP PDU tx (as in
response) was successful when the pdu was not transmitted by
egress slave for various reasons (including resource contention
within NIC) and only way to trace its fate is by looking at
ofproto->stats.tx_[packets/bytes] and slave port stats.

Adding a warning when there is tx failure could help user
debug at the root of this problem.

Signed-off-by: Gowrishankar Muthukrishnan <gmuthukr@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

ofproto/ofproto-dpif.c

@@ -3405,7 +3405,11 @@ send_pdu_cb(void *port_, const void *pdu, size_t pdu_size)
                                  pdu_size);
         memcpy(packet_pdu, pdu, pdu_size);
 
-        ofproto_dpif_send_packet(port, false, &packet);
+        error = ofproto_dpif_send_packet(port, false, &packet);
+        if (error) {
+            VLOG_WARN_RL(&rl, "port %s: cannot transmit LACP PDU (%s).",
+                         port->bundle->name, ovs_strerror(error));
+        }
         dp_packet_uninit(&packet);
     } else {
         static struct vlog_rate_limit rll = VLOG_RATE_LIMIT_INIT(1, 10);