mlxsw: core: Convert fwd_entry_ref list to be generic per-block resource list

Since the resource list needs to be used also for other entries different
to fwd_entry_ref, make the list generic. For that purpose, introduce a
resource structure with couple of helpers that the code which need to
store a per-block resource should use.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>