bridge: mcast: Place netlink policy before validation functions

Subsequent patches are going to add additional validation functions and
netlink policies. Some of these functions will need to perform parsing
using nla_parse_nested() and the new policies.

In order to keep all the policies next to each other, move the current
policy to before the validation functions.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>