container_of() is safe to use in regard to alignment requirements, when used
correctly. Using `uintptr_t` instead of `char *` for applying the offset results
in -Wcast-align not complaining.
IS_ACTIVE allows to evaluate macro definitions in non-preprocessor
expressions. It takes a macro that may be defined to 1 or not defined at
all and expands to 1 or 0 respectively.