kbuild: introduce __init_refok/__initdata_refok to supress section mismatch warnings

Throughout the kernel there are a few legitimite references
to init or exit sections. Most of these are covered by the
patterns included in modpost but a few nees special attention.
To avoid hardcoding a lot of function names in modpost introduce
a marker so relevant function/data can be marked.
When modpost see a reference to a init/exit function from
a function/data marked no warning will be issued.

Idea from: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>