ftp: Fix root path validation for upload directory

The ftp door can be configured to use a static root rather than the user's
root path. If the static root is not a prefix of the user's root directory,
the door should only allow access to the upload directory. The logic that
enforces this was broken, resulting in such upload attempts failing.

Target: trunk
Request: 2.10
Request: 2.9
Require-notes: yes
Require-book: no
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: https://rb.dcache.org/r/7347/
(cherry picked from commit 1321d431e155f11cf1d2e3665f3ac3265ddc9de8)
Merge pull request #875 from gbehrmann/fix/2.10/rb7347

ftp: Fix root path validation for upload directory