bonding: get rid of bond_option_active_slave_get()

Only keep bond_option_active_slave_get_rcu() helper.

bond_fill_info() uses a new bond_option_active_slave_get_ifindex()
helper.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Veaceslav Falico <vfalico@gmail.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>