bridge: netlink: account for the IFLA_BRPORT_PROXYARP attribute size and policy

The attribute size wasn't accounted for in the get_slave_size() callback
(br_port_get_slave_size) when it was introduced, so fix it now. Also add
a policy entry for it in br_port_policy.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Fixes: 95850116 ("bridge: Add support for IEEE 802.11 Proxy ARP")
Signed-off-by: David S. Miller <davem@davemloft.net>