TempReservationManager: Reserved() doesn't work

Note: It appears this has been around since the initial checkin of
TemporaryReservationManager. I have no idea what this could break, so
someone else may want to test this more thoroughly.

Signed-off-by: David Knowles <dknowles@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>