rpl: wrong dodag assignment to parent

`dodag` is the representation of the incoming DIO and thus is temporary.
Assigning this to the parent would lead to failure when reading/writing
to/from the parent's dodag attribute later.
`my_dodag`on the other hand is the local persistent representation of
the DIO's dodag and therefore the right choice.

I also had to move the code a little further down, because the parent uses the dodag's lifetime values to compute it's own lifetime.