xen-netback: implement dynamic multicast control

xen-netback: implement dynamic multicast control

My recent patch to the Xen Project documents a protocol for 'dynamic
multicast control' in netif.h. This extends the previous multicast control
protocol to not require a shared ring reconnection to turn the feature off.
Instead the backend watches the "request-multicast-control" key in xenstore
and turns the feature off if the key value is written to zero.

This patch adds support for dynamic multicast control in xen-netback.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 22fae97d863679994b951799dd4bbe7afd95897b)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
OraBug: 23268939