ocfs2: Optionally return filldir errors

Modify ocfs2_dir_foreach_blk() to optionally return any error from the
filldir callback. This way ocfs2_dirforeach() can terminate early, as
opposed to always passing through the entire directory. This fixes a bug
introduced during a previous code refactor where ocfs2_empty_dir() would
loop infinitely.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>