Replace XFS_ITOV_NULL() with the new VFS_I() inline.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31722a
Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>