afs/VNOPS: use osi_AllocSmallSpace() correctly

Send the size down to the allocator so it can perform some
consistency checks.

Reviewed-on: http://gerrit.openafs.org/11168
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit 2d973c10f13b40c0ac9da25fe7f4c73b3560a686)

Change-Id: I38d16bdd014010a2ccc9636b890c9ea7a6158e6a
Reviewed-on: http://gerrit.openafs.org/11203
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>