[XFS] Avoid directly referencing the VFS inode.

In several places we directly convert from the XFS inode
to the linux (VFS) inode by a simple deference of ip->i_vnode.
We should not do this - a helper function should be used to
extract the VFS inode from the XFS inode.

Introduce the function VFS_I() to extract the VFS inode
from the XFS inode. The name was chosen to match XFS_I() which
is used to extract the XFS inode from the VFS inode.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31720a

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>