bridge: add MDB get uAPI attributes

Add MDB get attributes that correspond to the MDB set attributes used in
RTM_NEWMDB messages. Specifically, add 'MDBA_GET_ENTRY' which will hold
a 'struct br_mdb_entry' and 'MDBA_GET_ENTRY_ATTRS' which will hold
'MDBE_ATTR_*' attributes that are used as indexes (source IP and source
VNI).

An example request will look as follows:

[ struct nlmsghdr ]
[ struct br_port_msg ]
[ MDBA_GET_ENTRY ]
	struct br_mdb_entry
[ MDBA_GET_ENTRY_ATTRS ]
	[ MDBE_ATTR_SOURCE ]
		struct in_addr / struct in6_addr
	[ MDBE_ATTR_SRC_VNI ]
		u32

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