Fix clients possibly not destroyed after being switched to RDMA

It could in theory lead to hung operations on RDMA client disconnections.

Workflow (probable):
* OSD switches an outbound connection to PEER_RDMA and removes the FD from epoll
* But a receive request in io_uring is still active
* Receive response is never handled, refs remains > 0
* The connection is dropped, but osd_client_t is not destroyed because refs > 0
* Outbound ops are not canceled, suboperations hang, primary operations hang too

src/client/msgr_rdma.cpp

@@ -696,6 +696,10 @@ void osd_messenger_t::handle_rdma_events(msgr_rdma_context_t *rdma_context)
                 continue;
             }
             osd_client_t *cl = cl_it->second;
+            if (cl->peer_state == PEER_STOPPED)
+            {
+                continue;
+            }
             auto rc = cl->rdma_conn;
             if (wc[i].status != IBV_WC_SUCCESS)
             {