srm: Fix logging of DN to access log

Regression compared to 2.10.

Target: trunk
Request: 2.11
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7419/
(cherry picked from commit 6c6e49698e2aa6f1911145d616a309f52f007728)