[XFS] remove i_gen from incore inode

i_gen is incremented in directory operations when the
directory is changed. It is never read or otherwise used
so it should be removed to help reduce the size of the
struct xfs_inode.

The patch also removes a duplicate logging of the directory
inode core. We only need to do this once per transaction
so kill the one associated with the i_gen increment.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Niv Sardi <xaiki@sgi.com>