pioctl.c: removed unused variable

pioctl.c: removed unused variable

The 'rval' variable is only actually used in the LINUX20 case;
adding another conditional block is making the LINUX20 case
different enough that it should get split out entirely.
Doing so lets the 'else' clause be simpler.

Found by clang on FreeBSD 10.0.

Reviewed-on: http://gerrit.openafs.org/10819
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit b9fb9c62a6779aa997259ddf2a83a90b08e04d5f)

Change-Id: I47f781bc13d54ad5a1b34365fcb9680793b206d1
Reviewed-on: http://gerrit.openafs.org/11778
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>