introduce a parallel variant of ->iterate()

New method: ->iterate_shared().  Same arguments as in ->iterate(),
called with the directory locked only shared.  Once all filesystems
switch, the old one will be gone.

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

fs/exportfs/expfs.c

@@ -312,7 +312,7 @@ static int get_name(const struct path *path, char *name, struct dentry *child)
 		goto out;
 
 	error = -EINVAL;
-	if (!file->f_op->iterate)
+	if (!file->f_op->iterate && !file->f_op->iterate_shared)
 		goto out_close;
 
 	buffer.sequence = 0;