[XFS] simplify vn_revalidate

No need to allocate a bhv_vattr_t on stack and call xfs_getattr to update
a few fields in the Linux inode from the XFS inode, just do it directly.

And yes, this function is in dire need of a better name and prototype,
I'll do in a separate patch, though.

SGI-PV: 970705
SGI-Modid: xfs-linux-melb:xfs-kern:29713a

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