net/core: Introduce netdev_get_xmit_slave

Add new ndo to get the xmit slave of master device. The reference
counters are not incremented so the caller must be careful with locks.
User can ask to get the xmit slave assume all the slaves can
transmit by set all_slaves arg to true.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>