bridge: Allow setting multicast_fast_leave_option

Setting the multicast_fast_leave option of a bridge allows to control
the forwarding of multicast traffic when an IGMP/MLD leave is received.
In case multicast_leave_option is enabled and a leave is received the
multicast membership will immediately be dropped on the bridge port while
in the other case the multicast membership will time out in the bridge.

This could be usefull in scenarios where explicit multicast membership
host tracking is not supported in the upstream network. In this case the
multicast stream is still flowing after a leave is received resulting into
possible bandwidth saturation on the lan if a new stream is joined as
multiple multicast streams are received.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>