rtnetlink: Add MDB get support

Now that both the bridge and VXLAN drivers implement the MDB get net
device operation, expose the functionality to user space by registering
a handler for RTM_GETMDB messages. Derive the net device from the
ifindex specified in the ancillary header and invoke its MDB get NDO.

Note that unlike other get handlers, the allocation of the skb
containing the response is not performed in the common rtnetlink code as
the size is variable and needs to be determined by the respective
driver.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>