bonding: Add function to get the xmit slave in active-backup mode

Add helper function to get the xmit slave in active-backup mode.
It's only one line function that return the curr_active_slave,
but it will used both in the xmit flow and by the new .ndo to get
the xmit slave.

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