libafs: allocate vattrs in LINUX to reduce stack used

Allocate temporary vattrs in LINUX to reduce the amount
of stack space used.

Reviewed-on: http://gerrit.openafs.org/11170
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>
(cherry picked from commit 40fb2650b783fbafe51aefd3d0af7a6b0536c265)

Change-Id: I1f19c07a2ad0f8cdddd88e4528584caccaa403e0
Reviewed-on: http://gerrit.openafs.org/11339
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>