add support for 'exact-len' validation on netlink attributes.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/340Acked-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231023-send-net-next-20231023-1-v2-2-16b1f701f900@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>