Add calls to KIO::UDSEntry::reserve in timeline/tags ioslaves

Summary: Minor optimization

Test Plan: make

Reviewers: Baloo, Frameworks, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D15996