drbd: add YNL genetlink specification

Add a YNL-conformant YAML spec for the DRBD genetlink interface.

This uses the genetlink-legacy schema since DRBD has a fixed protocol
header (drbd_genlmsghdr) and pre-existing conventions that userspace
depends on.

s32 fields (meta-dev-idx, resync-after) use nla-policy-type: u32 to
preserve wire compatibility with existing userspace; the original spec
predates kernel NLA_S32 support.

Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>

Author: chrboe