dup_iter
p->to_free = dup_iter(&p->data, to, GFP_KERNEL);
priv->to_free = dup_iter(&priv->to, to, GFP_KERNEL);
cmd->read_iov = dup_iter(cmd->read_iter, iter, GFP_KERNEL);
const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags);
EXPORT_SYMBOL(dup_iter);