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.

Change-Id: I8491117ead626e70aac40343923d52284f274efd
Reviewed-on: https://gerrit.openafs.org/12315
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>