vfs: use ERR_CAST for err-ptr tossing in lookup_instantiate_filp

Replace unclear (struct dentry *) to (struct file *) typecast with ERR_CAST() macro.
Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/open.c

@@ -793,7 +793,7 @@ struct file *lookup_instantiate_filp(struct nameidata *nd, struct dentry *dentry
 	return nd->intent.open.file;
 out_err:
 	release_open_intent(nd);
-	nd->intent.open.file = (struct file *)dentry;
+	nd->intent.open.file = ERR_CAST(dentry);
 	goto out;
 }
 EXPORT_SYMBOL_GPL(lookup_instantiate_filp);