bonding: simplify broadcast_xmit function

We don't need to start from the curr_active_slave as the frame will be
sent to all eligible slaves anyway, so we remove the unnecessary local
variables, checks and comments, and make it use the standard list API.
This has the nice side-effect that later when it's converted to RCU
a race condition will be avoided which could lead to double packet tx.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>