libafs: api to create and free vattrs

Add a pair of functions to allocate and free struct vattrs,
to avoid having struct vattrs on the stack.

Reviewed-on: http://gerrit.openafs.org/11169
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit 9630c075409a262424805411ed473178814b412c)

Change-Id: I7746ee69f7fde8760c066458ecfe44da46edbf81
Reviewed-on: http://gerrit.openafs.org/11338
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
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>