If we read zero bytes from the directory, early out with ENOENT

rather than forging ahead and interpreting garbage buffer content
and dirent structures.

This change backs out r211684 which was essentially a no-op.

MFC after:	1 week

sys/kern/vfs_default.c

@@ -281,9 +281,13 @@ get_next_dirent(struct vnode *vp, struct dirent **dpp, char *dirbuf,
 		if (error)
 			return (error);
 
-		*cpos = dirbuf;
-		*len = uio.uio_offset - *off;
 		*off = uio.uio_offset;
+
+		*cpos = dirbuf;
+		*len = (dirbuflen - uio.uio_resid);
+
+		if (*len == 0)
+			return (ENOENT);
 	}
 
 	dp = (struct dirent *)(*cpos);