bridge: mdb: add support for more attributes and export timer

bridge: mdb: add support for more attributes and export timer

Currently mdb entries are exported directly as a structure inside
MDBA_MDB_ENTRY_INFO attribute, we can't really extend it without
breaking user-space. In order to export new mdb fields, I've converted
the MDBA_MDB_ENTRY_INFO into a nested attribute which starts like before
with struct br_mdb_entry (without header, as it's casted directly in
iproute2) and continues with MDBA_MDB_EATTR_ attributes. This way we
keep compatibility with older users and can export new data.
I've tested this with iproute2, both with and without support for the
added attribute and it works fine.
So basically we again have MDBA_MDB_ENTRY_INFO with struct br_mdb_entry
inside but it may contain also some additional MDBA_MDB_EATTR_ attributes
such as MDBA_MDB_EATTR_TIMER which can be parsed by user-space.

So the new structure is:
[MDBA_MDB] = {
     [MDBA_MDB_ENTRY] = {
         [MDBA_MDB_ENTRY_INFO]
         [MDBA_MDB_ENTRY_INFO] { <- Nested attribute
             struct br_mdb_entry <- nla_put_nohdr()
             [MDBA_MDB_ENTRY attributes] <- normal netlink attributes
         }
     }
}

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>