bridge: mcast: Centralize netlink attribute parsing

Netlink attributes are currently passed deep in the MDB creation call
chain, making it difficult to add new attributes. In addition, some
validity checks are performed under the multicast lock although they can
be performed before it is ever acquired.

As a first step towards solving these issues, parse the RTM_{NEW,DEL}MDB
messages into a configuration structure, relieving other functions from
the need to handle raw netlink attributes.

Subsequent patches will convert the MDB code to use this configuration
structure.

This is consistent with how other rtnetlink objects are handled, such as
routes and nexthops.

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