ocfs2: Implement ocfs2_empty_dir() as a caller of ocfs2_dir_foreach()

We can preserve the behavior of ocfs2_empty_dir(), while getting rid of the
open coded directory walk by just providing a smart filldir callback. This
also automatically gets to use the dir readahead code, though in this case
any advantage is minor at best.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>
Reviewed-by: Joel Becker <joel.becker@oracle.com>