lib/nfs/FileReader: postpone the nfs_close_async() call

If an async opertion is in progress, nfs_close_async() will make
libnfs crash because the RPC callback will dereference an object that
was freed by nfs_close_async().

src/lib/nfs/Cancellable.hxx

@@ -150,6 +150,13 @@ public:
 
 		i->Cancel();
 	}
+
+	CT &Get(reference_type p) {
+		auto i = Find(p);
+		assert(i != list.end());
+
+		return *i;
+	}
 };
 
 #endif