RDMA/cm: Fix leaking the multicast GID table reference

If the CM ID is destroyed while the CM event for multicast creating is
still queued the cancel_work_sync() will prevent the work from running
which also prevents destroying the ah_attr. This leaks a refcount and
triggers a WARN:

GID entry ref leak for dev syz1 index 2 ref=573
WARNING: CPU: 1 PID: 655 at drivers/infiniband/core/cache.c:809 release_gid_table drivers/infiniband/core/cache.c:806 [inline]
WARNING: CPU: 1 PID: 655 at drivers/infiniband/core/cache.c:809 gid_table_release_one+0x284/0x3cc drivers/infiniband/core/cache.c:886

Destroy the ah_attr after canceling the work, it is safe to call this
twice.

Link: https://patch.msgid.link/r/[e-mail]
Cc: [e-mail]
Fixes: fe454dc31e84 ("RDMA/ucma: Fix use-after-free bug in ucma_create_uevent")
Reported-by: [e-mail]
Closes: https://lore.kernel.org/all/[e-mail]
Signed-off-by: Jason Gunthorpe <[e-mail]>

drivers/infiniband/core/cma.c

···
 		ib_sa_free_multicast(mc->sa_mc);
 
 	if (rdma_protocol_roce(id_priv->id.device, id_priv->id.port_num)) {
+		struct rdma_cm_event *event = &mc->iboe_join.event;
 		struct rdma_dev_addr *dev_addr =
 			&id_priv->id.route.addr.dev_addr;
 		struct net_device *ndev = NULL;
···
 		dev_put(ndev);
 
 		cancel_work_sync(&mc->iboe_join.work);
+		if (event->event == RDMA_CM_EVENT_MULTICAST_JOIN)
+			rdma_destroy_ah_attr(&event->param.ud.ah_attr);
 	}
 	kfree(mc);
 }