xfs: move struct xfs_attr_shortform to xfs_da_format.h

Move the shortform attr structure definition to the same place as the
other attribute structure definitions for consistency and also so that
xfs/122 verifies the structure size.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>