shared: make container_of() use unique variable names

If you stack container_of() macros, you will get warnings due to shadowing
variables of the parent context. To avoid this, use unique names for
variables.

Two new helpers are added:
  UNIQ: This evaluates to a truly unique value never returned by any
        evaluation of this macro. It's a shortcut for __COUNTER__.
  UNIQ_T: Takes two arguments and concatenates them. It is a shortcut for
          CONCATENATE, but meant to defined typed local variables.

As you usually want to use variables that you just defined, you need to
reference the same unique value at least two times. However, UNIQ returns
a new value on each evaluation, therefore, you have to pass the unique
values into the macro like this:

    #define my_macro(a, b) __max_macro(UNIQ, UNIQ, (a), (b))
    #define __my_macro(uniqa, uniqb, a, b) ({
                typeof(a) UNIQ_T(A, uniqa) = (a);
                typeof(b) UNIQ_T(B, uniqb) = (b);
                MY_UNSAFE_MACRO(UNIQ_T(A, uniqa), UNIQ_T(B, uniqb));
        })

This way, MY_UNSAFE_MACRO() can safely evaluate it's arguments multiple
times as they are local variables. But you can also stack invocations to
the macro my_macro() without clashing names.

This is the same as if you did:

    #define my_macro(a, b) __max_macro(__COUNTER__, __COUNTER__, (a), (b))
    #define __my_macro(prefixa, prefixb, a, b) ({
                typeof(a) CONCATENATE(A, prefixa) = (a);
                typeof(b) CONCATENATE(B, prefixb) = (b);
                MY_UNSAFE_MACRO(CONCATENATE(A, prefixa), CONCATENATE(B, prefixb));
        })

...but in my opinion, the first macro is easier to write and read.

This patch starts by converting container_of() to use this new helper.
Other macros may follow (like MIN, MAX, CLAMP, ...).

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