poolmanager: Fix infinite loop in lru partition type

Target: trunk
Require-notes: yes
Require-book: no
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6938/
(cherry picked from commit 6750fd426d7e60da7332453a28bfbc88d228274e)