fix negative disk space reservation content (#1120)

ReleaseNotes.md

common/dataStructures/DiskSpaceReservationRequest.cpp

common/dataStructures/DiskSpaceReservationRequest.hpp

scheduler/OStoreDB/OStoreDB.cpp

scheduler/OStoreDB/OStoreDB.hpp

scheduler/RetrieveMount.cpp

scheduler/RetrieveMount.hpp

scheduler/SchedulerDatabase.hpp

...ver/castor/tape/tapeserver/daemon/DataTransferSession.cpp

...erver/castor/tape/tapeserver/daemon/DiskWriteTaskTest.cpp

...castor/tape/tapeserver/daemon/DiskWriteThreadPoolTest.cpp

...rver/castor/tape/tapeserver/daemon/RecallTaskInjector.cpp

...rver/castor/tape/tapeserver/daemon/RecallTaskInjector.hpp

.../castor/tape/tapeserver/daemon/RecallTaskInjectorTest.cpp