Closes https://github.com/rails/rails/issues/45483
Closes https://github.com/rails/rails/pull/45523



To quote #45483:

> `field_name` is adding an extra index parameter, the extra `[0]` in
> `parent[children_attributes][0][0][grandchildren_attributes][]`

To resolve that issue, this commit reads its default `field_id` and
`field_name` method's `index:` option directly from the `@options`.
Prior to this commit, that value was read from the `@index` instance
variable.

Signed-off-by: Akira Matsuda <ronnie@dio.jp>