Coccinelle: add atomic_as_refcounter script

atomic_as_refcounter.cocci script allows detecting
cases when refcount_t type and API should be used
instead of atomic_t.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>