nfsd4: fix delegation recall race use-after-free

When the rarely-used callback-connection-changing setclientid occurs
simultaneously with a delegation recall, we rerun the recall by
requeueing it on a workqueue.  But we also need to take a reference on
the delegation in that case, since the delegation held by the rpc itself
will be released by the rpc_release callback.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfs4callback.c

@@ -689,6 +689,7 @@ static void nfsd4_cb_recall_done(struct rpc_task *task, void *calldata)
 		warn_no_callback_path(clp, task->tk_status);
 		if (current_rpc_client != task->tk_client) {
 			/* queue a callback on the new connection: */
+			atomic_inc(&dp->dl_count);
 			nfsd4_cb_recall(dp);
 			return;
 		}