wireguard: netlink: enable strict genetlink validation

wireguard: netlink: enable strict genetlink validation

WireGuard is a modern enough genetlink family, that it doesn't need
resv_start_op. It already had policies in place when it was first
merged, it has also never used the reserved field, or other things
toggled by resv_start_op.

wireguard-tools have always used zero initialized memory, and have never
touched the reserved field, neither have any other clients I have
checked. Closed-source clients are much more likely to use the
embeddedable library from wireguard-tools, than a DIY implementation
using uninitialized memory.

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