drbd: prepare the transition from connector to genetlink

This adds the new API header and helper files.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

diff --cc include/linux/drbd_genl.h
Simple merge

diff --cc include/linux/drbd_genl_api.h
Simple merge

diff --cc include/linux/genl_magic_func.h
Simple merge

diff --cc include/linux/genl_magic_struct.h
Simple merge