venus: fix memory leak

venus: fix memory leak

In GetPrefCmd, when we request server prefs from the kernel and our
output buffer is not big enough, pioctl() will return E2BIG and we
allocate more memory and try again. However, if the size of the output
buffer reaches 16k bytes and this space is still not enough (or if
pioctl fails and errno != E2BIG), we return without releasing the
memory that was previously allocated.

To fix this problem, free our output buffer when this happens.

Reviewed-on: https://gerrit.openafs.org/12293
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 8ad4e15ffc883c9a99f9636d7d8a5ed0a2fcc26a)

Change-Id: I62ceddc5284c94da205ec2351ab9ef970cd64c4a
Reviewed-on: https://gerrit.openafs.org/13895
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>