(please correct me if I didn't get the question right:)
I wouldn't do that way.
Instead of using the _check_login() function callback as a validator for the username field (which doesn't really make sense, imho), why not call the check_login($username,$password) function WHEN the input fields are validated?
So
if($this->form_validation->run() == FALSE)
{
$this->load->view('user_login');
}
else
{
$this->user_model->check_login();
}
In User model you will be doing the check
Anyway, that's what the form_validation library is for, to validate inputs, not to elaborate datas.
If you just wanna check the existence of a user (to avoid duplicates, for example) to validate the input field, then you don't need the $password parameter, so the callback would work just fine.