bonding: fix to rejoin multicast groups immediately

The IGMP specs states that if the system receives a
membership report, it shouldn't send another for the
next minute. However, if a link failure happens right
after that, the backup slave and the switch connected
to this slave will not know about the multicast and
the traffic will hang for about a minute.

This patch fixes it to rejoin multicast groups immediately
after a failover restoring the multicast traffic.

Signed-off-by: Flavio Leitner <fleitner@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>