venus: fix memory leak

venus: fix memory leak

The fs getserverprefs command displays preference
ranks for file / volume location server machine
interfaces. In order to get the complete set of
preference ranks, the VIOC_GETSPREFS system call
might have to be called several times. If so, the
memory previously allocated should be released.

Reviewed-on: https://gerrit.openafs.org/12315
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit d3b8a05d229a80100f40fca4dfdcd820313fcea8)

Change-Id: Iccd9b98aea1c7cf1d63ad20c470baabe09f7a0d7
Reviewed-on: https://gerrit.openafs.org/12340
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>