[FileItemRolesUpdater] Fix reservation size

The code below always adds the visible range, up to 5 pages before and
after the visible range (each capped to ResolveAllItemsLimit/2) and
the very first and last page (each up to m_maximumVisibleItems).

Use this number to avoid growing the list later.
1 job for master in 7 minutes and 29 seconds (queued for 34 minutes and 57 seconds)
Status Job ID Name Coverage
  Build
passed #92251
linux kf5-qt5 qt5.15

00:07:29

52.0%