rtnetlink: add ndo_fdb_dump_context

rtnetlink: add ndo_fdb_dump_context

rtnl_fdb_dump() and various ndo_fdb_dump() helpers share
a hidden layout of cb->ctx.

Before switching rtnl_fdb_dump() to for_each_netdev_dump()
in the following patch, make this more explicit.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20241209100747.2269613-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>