Fix regression in message queue length reporting

Addresses the issue that the message queue length in cells
is always reported as zero. This regression was introduced
between dCache 1.9.5 and 1.9.12.

Target: trunk
Request: 2.6
Request: 2.2
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/5875/
(cherry picked from commit e45b78b7cd9293a74fa2b479c398fb93e8c412c0)
Merge pull request #146 from gbehrmann/fix/2.6/rb5875

Fix/2.6/rb5875