pool: Report correct client IP to billing for passive FTP transfers

The pool incorrectly reported the FTP proxy address of the FTP door
even when a direct connection between the client and the pool was
established.

I am undecided how far back to fix this. Please advice.

Target: trunk
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6940/
(cherry picked from commit 7e6e2ef7a8ad506293fc440cc1354eeed9c2607a)
Merge pull request #512 from gbehrmann/fix/2.9/rb6940

pool: Report correct client IP to billing for passive FTP transfers