libafs: reduce stack space

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

Reviewed-on: http://gerrit.openafs.org/11003
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 027e2bbb1191c31dbbe968a925c192b064d52c47)

Change-Id: I2dc6d49c7d3b7578359930f7fd922f177729281f
Reviewed-on: http://gerrit.openafs.org/11165
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Gergely Madarasz <madaraszg@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>