[BUGFIX] "New page" wizard discloses existence of pages outside DB mount

When creating a new page inside the top level of a DB mount which is
only a sub tree, the pages up and down from the DB mount root will be
displayed in the position selector if the logged-in user has read
permissions for these pages. This is unwanted information disclosure as
the permissions should not matter for pages which are outside the DB
mount.

Resolves: #18797
Releases: 6.2, 6.1, 6.0
Change-Id: I98008bc7f4308c9fb32dae645325e7cb1b44e413
Reviewed-on: https://review.typo3.org/27430
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind