[XFS] cleanup vnode useage in xfs_iget.c

Get rid of vnode useage in xfs_iget.c and pass Linux inode / xfs_inode
where apropinquate. And kill some useless helpers while we're at it.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:29808a

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