libafs: reduce stack space in VNOPS

Allocate temporary vrequests to reduce the amount
of stack space used.

Reviewed-on: http://gerrit.openafs.org/11004
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit 9930567bcf9655d3f562b210b2dc4b4a99226691)

Change-Id: I1c60701d4e4f85aedfea6be58a03468ab34edb96
Reviewed-on: http://gerrit.openafs.org/11166
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>