pnfsmanager: Fix permission check on root directory

Addresses an issue in which listing the root directory through
FTP fails with a null pointer exception.

The problem has previously been fixed on master, however the
merge to 2.2 was incomplete due to code difference between
the two branches.

Target: 2.2
Request: 2.2
Require-book: no
Require-notes: yes
Ticket: http://rt.dcache.org/Ticket/Display.html?id=7757
Patch: http://rb.dcache.org/r/5488/
Acked-by: Tigran Mkrtchyn <tigran.mkrtchyan@desy.de>