bonding: Implement ndo_get_xmit_slave

Add implementation of ndo_get_xmit_slave. Find the slave by using the
helper function according to the bond mode. If the caller set all_slaves
to true, then it assumes that all slaves are available to transmit.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>