Turn the validation_err() macro into a function

Turn the validation_err() macro into a function

validation_err() is an ugly macro with side effects and a goto in it.
At the cost of a few lines of code we can turn this into a function
where the side effects are explicit and ret is now explicitly set in
the main body of addr_validate_path_internal().

We get to a point where it is halfway possible to reason about the
convoluted control flow in this function.

ok inoguchi jsing