libafs: api to create and free vrequests

Add a pair of functions to allocate and free struct vrequests, which
are to be used to avoid having struct vrequests on the stack.

Reviewed-on: http://gerrit.openafs.org/11074
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit 76ad941902c650a4a716168d3cbe68f62aef109f)

Change-Id: I08932256af58aeba31b2d5c11008658c419cf008
Reviewed-on: http://gerrit.openafs.org/11164
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
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>