wireguard: netlink: convert to split ops

wireguard: netlink: convert to split ops

This patch converts WireGuard from using the legacy struct genl_ops
to struct genl_split_ops, by applying the same transformation as
genl_cmd_full_to_split() would otherwise do at runtime.

WGDEVICE_A_MAX is swapped for WGDEVICE_A_PEERS, while they are
currently equivalent, then .maxattr should be the maximum attribute
that a given command supports, and not change along with WGDEVICE_A_MAX.

This is an incremental step towards adopting netlink policy code
generated by ynl-gen, ensuring that the code and spec is aligned.

This is a trivial patch with no behavioural changes intended.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>