netlink: add NLA_POLICY_MAX_LEN macro

Similarly to NLA_POLICY_MIN_LEN, NLA_POLICY_MAX_LEN defines a policy
with a maximum length value.

The netlink generator for YAML specs has been extended accordingly.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20241029-b4-ovpn-v11-1-de4698c73a25@openvpn.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>