SUNRPC: The TCP back channel mustn't disappear while requests are outstanding

If there are TCP back channel requests being processed by the
server threads, then we should hold a reference to the transport
to ensure it doesn't get freed from underneath us.

Reported-by: Neil Brown <[e-mail]>
Fixes: 2ea24497a1b3 ("SUNRPC: RPC callbacks may be split across several..")
Signed-off-by: Trond Myklebust <[e-mail]>
Signed-off-by: Anna Schumaker <[e-mail]>

net/sunrpc/backchannel_rqst.c

···
 		 */
 		dprintk("RPC:       Last session removed req=%p\n", req);
 		xprt_free_allocation(req);
-		return;
 	}
+	xprt_put(xprt);
 }
 
 /*
···
 		spin_unlock(&xprt->bc_pa_lock);
 		if (new) {
 			if (req != new)
-				xprt_free_bc_rqst(new);
+				xprt_free_allocation(new);
 			break;
 		} else if (req)
 			break;
···
 	set_bit(RPC_BC_PA_IN_USE, &req->rq_bc_pa_state);
 
 	dprintk("RPC:       add callback request to list\n");
+	xprt_get(xprt);
 	spin_lock(&bc_serv->sv_cb_lock);
 	list_add(&req->rq_bc_list, &bc_serv->sv_cb_list);
 	wake_up(&bc_serv->sv_cb_waitq);