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.

Change-Id: Ia5a148ebcdf8a2f1e3a2aa9d8fd3c0e0cd0e25e9
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>
libafs: allocate vattrs in LINUX to reduce stack used

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

Change-Id: Iafa8d920b7149486f1ea8fb1999c1f4c9a935615
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>