bonding: register slave pointer for rx_handler

Register slave pointer as rx_handler data. That would eventually prevent
need to loop over slave devices to find the right slave.

Use synchronize_net to ensure that bond_handle_frame does not get slave
structure freed when working with that.

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Reviewed-by: Nicolas de Pesloüan <nicolas.2p.debian@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>