spacemanager: Fix setter for cleanupExpiredSpaceFiles property

The wonders of static analysis....

Target: trunk
Require-notes: no
Require-book: no
Request: 2.7
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6213/
(cherry picked from commit 7e448d0197a971f3619008fbd98e185009174dcc)