UCT/RC/MLX5: Fix keepalive send condition with CQ moderation

- Send FC grant message (which is also used for keepalive) with CQ
  signaling enabled.
- Skip sending a keepalive message only if there are no unsignaled
  sends: if there are unsignaled sends, they could be completed already,
  so skipping a keepalive message could fail to detect a dead
  connection.

src/uct/ib/rc/accel/rc_mlx5_ep.c

@@ -589,7 +589,7 @@ ucs_status_t uct_rc_mlx5_ep_fc_ctrl(uct_ep_t *tl_ep, unsigned op,
                                  NULL, 0,
                                  UCT_RC_EP_FC_PURE_GRANT, 0, 0,
                                  0, 0,
-                                 NULL, NULL, 0, 0,
+                                 NULL, NULL, 0, MLX5_WQE_CTRL_CQ_UPDATE,
                                  INT_MAX);
     return UCS_OK;
 }

src/uct/ib/rc/accel/rc_mlx5_iface.c

@@ -173,10 +173,13 @@ uct_rc_mlx5_common_ka_progress(uct_rc_mlx5_iface_common_t *iface)
 
     ucs_spin_lock(&iface->super.ep_list_lock);
     ucs_list_for_each(ep, &iface->super.ep_list, super.list) {
-        if (ep->super.txqp.available < ep->tx.wq.bb_max) {
-            /* have outstanding operations */
+        if ((ep->super.txqp.available < ep->tx.wq.bb_max) &&
+            (ep->super.txqp.unsignaled == 0)) {
+            /* Have outstanding uncompleted operations - no need to send
+               keepalive message */
             continue;
         }
+
         ucs_trace("send keepalive grant on ep %p", ep);
         uct_rc_ep_fc_send_grant(&ep->super);
     }