bonding: extend round-robin mode with packets_per_slave

This patch aims to extend round-robin mode with a new option called
packets_per_slave which can have the following values and effects:
0 - choose a random slave
1 (default) - standard round-robin, 1 packet per slave
 >1 - round-robin when >1 packets have been transmitted per slave
The allowed values are between 0 and 65535.
This patch also fixes the comment style in bond_xmit_roundrobin().

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