xfs: rearrange the logic and remove the broken comment for xfs_dir2_isxx

xfs_dir2_isleaf is used to see if the directory is a single-leaf
form directory instead, as commented right above the function.

Besides getting rid of the broken comment, we rearrange the logic by
converting everything over to standard formatting and conventions,
at the same time, to make it easier to understand and self documenting.

Signed-off-by: Shida Zhang <zhangshida@kylinos.cn>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>