libafs: reduce stack space in VNOPS

libafs: reduce stack space in VNOPS

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

Change-Id: Ic14cc4f657f7c7e97ef396601bd6c8c7f91abe55
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>