After boot, the kernel frees up a special section; functions
marked with __init and data structures marked with
__initdata are dropped after boot is complete: similarly
modules discard this memory after initialization. __exit
is used to declare a function which is only required on exit: the
function will be dropped if this file is not compiled as a module.
See the header file for use. Note that it makes no sense for a function
marked with __init to be exported to modules with
EXPORT_SYMBOL() - this will break.