bonding: Add array of all slaves

Keep all slaves in array so it could be used to get the xmit slave
assume all the slaves are active.
The logic to add slave to the array is like the usable slaves, except
that we also add slaves that currently can't transmit - not up or active.

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>