libafs: reduce stack space in LINUX

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

Reviewed-on: http://gerrit.openafs.org/11005
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 1ac4eb1b84db2732b9c2645492daff9e25417475)

Change-Id: I5d982afce489fbcf82cbcb69bd2677fcb0236237
Reviewed-on: http://gerrit.openafs.org/11167
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Gergely Madarasz <madaraszg@gmail.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>