[BUGFIX] mounts of same storage had same subfolder count in Filelist

When you opened the filelist module as user who just was allowed to see
a bunch of filemounts, the filemounts had either all or none the expand-
icon, based on the first file-mount of this storage the user has access
to.

Resolves: #48467
Releases: 6.2, 6.1, 6.0
Change-Id: I1e6a29e351a643f52c0f7a42fde38c9d9b2fc5cb
Reviewed-on: https://review.typo3.org/20981
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind