[XFS] simplify xfs_vn_getattr

Just fill in struct kstat directly from the xfs_inode instead of doing a
detour through a bhv_vattr_t and xfs_getattr.

SGI-PV: 970980
SGI-Modid: xfs-linux-melb:xfs-kern:29770a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>