fstrace: only declare 'rval' when it is used

... to avoid compiler warnings about unused variables.

Found by clang on FreeBSD 10.0.

Change-Id: I30c9fbc0e2fb0f9af273f0f17861009b6d0577df
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>