[XFS] Dynamically allocate vattr in places it makes sense to do so, to

reduce stack use.  Also re-use vattr in some places so that multiple
copies are not held on-stack.

SGI-PV: 947312
SGI-Modid: xfs-linux-melb:xfs-kern:25369a

Signed-off-by: Nathan Scott <nathans@sgi.com>