netkit: Simplify netkit mode over to use NLA_POLICY_MAX

Jakub suggested to rely on netlink policy validation via NLA_POLICY_MAX()
instead of open-coding it. netkit_check_mode() is a candidate which can
be simplified through this as well aside from the netkit scrubbing one.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Nikolay Aleksandrov <razor@blackwall.org>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20241004101335.117711-2-daniel@iogearbox.net


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>