xstat: fix a malformed debug message

a munged format specifier obscured the missing rn arg.

Reviewed-on: http://gerrit.openafs.org/10708
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit ba0208f1d69c8c403150fb6312214f9b1c972c6d)

Change-Id: Ie422f4a7be98052fec9281be73eac1823e9ea57f
Reviewed-on: http://gerrit.openafs.org/11154
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>