libafs: reduce stack space

libafs: reduce stack space

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

Change-Id: I8c50a3af3028512003a02e46a2960b9b135213a5
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>