mlxsw: spectrum: Keep mirror netdev in mlxsw_sp_span_entry

Currently the only mirror action supported by mlxsw is mirror to another
mlxsw physical port. Correspondingly, span_entry, which tracks each
mlxsw mirror in the system, currently holds a u8 number of the
destination port.

To extend this system to mirror to gretap and ip6gretap netdevices, have
struct mlxsw_sp_span_entry actually hold the destination netdevice
itself.

This change then trickles down in obvious manner to SPAN module API and
mirror-related interfaces in struct mlxsw_afa_ops.

To prevent use of invalid pointer, NETDEV_UNREGISTER needs to be hooked
and the corresponding SPAN entry invalidated.

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