bonding: rejoin multicast groups on VLANs

During a failover, the IGMP membership is sent to update
the switch restoring the traffic, but it misses groups added
to VLAN devices running on top of bonding devices.

This patch changes it to iterate over all VLAN devices
on top of it sending IGMP memberships too.

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