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.

Change-Id: I60c56af355fdb68752d9596ff2cd7a4259b43fe9
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>