IB/cma: Do not send IGMP leaves for sendonly Multicast groups

IB/cma: Do not send IGMP leaves for sendonly Multicast groups

ROCE uses IGMP for Multicast instead of the native Infiniband system where
joins are required in order to post messages on the Multicast group.  On
Ethernet one can send Multicast messages to arbitrary addresses without
the need to subscribe to a group.

So ROCE correctly does not send IGMP joins during rdma_join_multicast().

F.e. in cma_iboe_join_multicast() we see:

   if (addr->sa_family == AF_INET) {
                if (gid_type == IB_GID_TYPE_ROCE_UDP_ENCAP) {
                        ib.rec.hop_limit = IPV6_DEFAULT_HOPLIMIT;
                        if (!send_only) {
                                err = cma_igmp_send(ndev, &ib.rec.mgid,
                                                    true);
                        }
                }
        } else {

So the IGMP join is suppressed as it is unnecessary.

However no such check is done in destroy_mc(). And therefore leaving a
sendonly multicast group will send an IGMP leave.

This means that the following scenario can lead to a multicast receiver
unexpectedly being unsubscribed from a MC group:

1. Sender thread does a sendonly join on MC group X. No IGMP join
   is sent.

2. Receiver thread does a regular join on the same MC Group x.
   IGMP join is sent and the receiver begins to get messages.

3. Sender thread terminates and destroys MC group X.
   IGMP leave is sent and the receiver no longer receives data.

This patch adds the same logic for sendonly joins to destroy_mc() that is
also used in cma_iboe_join_multicast().

Fixes: 0ab16f349771 ("IB/core: Support for CMA multicast join flags")
Link: https://lore.kernel.org/r/alpine.DEB.2.22.394.2109081340540.668072@gentwo.de
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>