afs/VNOPS: reduce stack usage

AFSFetchStatus is rather large and is in the stack twice.  Allocating with
osi_AllocSmallSpace will save about 160 bytes of stack.

Change-Id: I13ac31814d9d5975f245ba8c66a7befac471503e
Reviewed-on: http://gerrit.openafs.org/10964
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>