[XFS] no longer using io_vnode, as was remaining from 23 cherrypick

Because we cherrypicked SGI-Modid xfs-linux-melb:xfs-kern:29675a
and it depended on the sgi mod which removed io_vnode (which was
not cherrypicked in 23) it was hand modified.
This fixes things back up (to the originial mod) now we have moved
on again.

Reviewed-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>