[readdir] introduce iterate_dir() and dir_context

iterate_dir(): new helper, replacing vfs_readdir().

struct dir_context: contains the readdir callback (and will get more stuff
in it), embedded into whatever data that callback wants to deal with;
eventually, we'll be passing it to ->readdir() replacement instead of
(data,filldir) pair.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/parisc/hpux/fs.c

@@ -60,6 +60,7 @@ struct hpux_dirent {
 };
 
 struct getdents_callback {
+	struct dir_context ctx;
 	struct hpux_dirent __user *current_dir;
 	struct hpux_dirent __user *previous;
 	int count;
@@ -121,8 +122,9 @@ int hpux_getdents(unsigned int fd, struct hpux_dirent __user *dirent, unsigned i
 	buf.previous = NULL;
 	buf.count = count;
 	buf.error = 0;
+	buf.ctx.actor = filldir;
 
-	error = vfs_readdir(arg.file, filldir, &buf);
+	error = iterate_dir(arg.file, &buf.ctx);
 	if (error >= 0)
 		error = buf.error;
 	lastdirent = buf.previous;