fstrace: only declare 'rval' when it is used

fstrace: only declare 'rval' when it is used

... to avoid compiler warnings about unused variables.

Found by clang on FreeBSD 10.0.

Reviewed-on: http://gerrit.openafs.org/10822
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 63291be2216762dd89072f41c9a016608b736ceb)

Change-Id: Ib5d7e14d6077ec2377180b9308d99f49ff79cccc
Reviewed-on: http://gerrit.openafs.org/11777
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>