bridge: mcast: Move validation to a policy

Future patches are going to move parts of the bridge MDB code to the
common rtnetlink code in preparation for VXLAN MDB support. To
facilitate code sharing between both drivers, move the validation of the
top level attributes in RTM_{NEW,DEL}MDB messages to a policy that will
eventually be moved to the rtnetlink code.

Use 'NLA_NESTED' for 'MDBA_SET_ENTRY_ATTRS' instead of
NLA_POLICY_NESTED() as this attribute is going to be validated using
different policies in the underlying drivers.

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