xfs: move di_changecount to VFS inode

We can store the di_changecount in the i_version field of the VFS
inode and remove another 8 bytes from the xfs_icdinode.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>