RDMA/cma: Initialize ib_sa_multicast structure to 0 when join

Initialize the structure to 0 so that it's fields won't have random
values. For example fields like rec.traffic_class (as well as
rec.flow_label and rec.sl) is used to generate the user AH through:
  cma_iboe_join_multicast
    cma_make_mc_event
      ib_init_ah_from_mcmember

And a random traffic_class causes a random IP DSCP in RoCEv2.

Fixes: b5de0c6 ("RDMA/cma: Fix use after free race in roce multicast join")
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Link: https://lore.kernel.org/r/20230927090511.603595-1-markzhang@nvidia.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>

Author: MarkZhang81

drivers/infiniband/core/cma.c

@@ -4968,7 +4968,7 @@ static int cma_iboe_join_multicast(struct rdma_id_private *id_priv,
 	int err = 0;
 	struct sockaddr *addr = (struct sockaddr *)&mc->addr;
 	struct net_device *ndev = NULL;
-	struct ib_sa_multicast ib;
+	struct ib_sa_multicast ib = {};
 	enum ib_gid_type gid_type;
 	bool send_only;