mctp: implement asynchronous polling for bridge downstream EIDs

Add asynchronous polling mechanism for MCTP bridge downstream endpoints
to discover them. The polling continues throughout the bridge's existence
under the MCTP network and creates peer structures only when endpoints respond
to poll successfully.

- Update bus-owner config with polling related factors such as
  ep_removal_threshold, poll_timeout
- Add bridge_eid_poll_context and bridge_endpoint_discovery structures
- Implement setup_async_downstream_ep_poll() for polling initialization
- Add send_bridge_poll_request() for periodic GET_ENDPOINT_ID messages
- Add handle_bridge_poll_response() for response processing
- Add handle_poll_timeout() with ep_removal_threshold failure handling
- Add cleanup_bridge_polling() and cleanup_bridge_downstream_peers()

The polling uses an asynchronous sender-response model where mctpd
continuously polls downstream EIDs and creates peer structures upon
successful responses. Failed endpoints are removed after reaching
the failure threshold (ep_removal_threshold). Polling stops and all resources
are cleaned up when bridges are removed.

Signed-off-by: Faizan Ali <[email protected]>

Author: faizana-nvidia

conf/mctpd.conf

@@ -8,6 +8,9 @@ message_timeout_ms = 30
 # bus-owner configuration
 [bus-owner]
 max_pool_size = 15
+ep_removal_thresold = 3
+mctp_treclaim_interval_us = 5000000 # 5s
+poll_timeout_us = 400000 # 400ms MT2
 
 # Specify a UUID: not generally required - mctpd will query the system UUID
 # where available.

src/mctp-control-spec.h

@@ -330,3 +330,8 @@ struct mctp_ctrl_resp_alloc_eid {
 #define GET_ROUTING_ENTRY_TYPE(field)                 \
 	(((field) >> MCTP_ROUTING_ENTRY_TYPE_SHIFT) & \
 	 MCTP_ROUTING_ENTRY_TYPE_MASK)
+
+/* MCTP Time Specification
+ * See DSP0283 v1.0.0 Table 4
+ */
+#define MCTP_TRECLAIM_US 5000000 // 5s