xfs: compat_xfs_bstat does not have forkoff

struct compat_xfs_bstat is missing the di_forkoff field and so does
not fully translate the structure correctly. Fix it.

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