[XFS] XFS: Kill xfs_vtoi()

xfs_vtoi() is redundant and only unsed in small sections of code.
Replace them with widely used XFS_I() inline and kill xfs_vtoi().

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31725a

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>