RDMA/mlx4: Restrict external umem for CQ when copy_to_user() is used

When the mlx4 firmware reports the MLX4_DEV_CAP_FLAG2_SW_CQ_INIT capability,
libmlx4 from the rdma-core package expects the driver to initialize memory
at the address provided in the buf_addr parameter of ucmd.

This behavior cannot be supported by any external umem implementation, so
restrict it accordingly.

Fixes: f45f195af5 ("RDMA/mlx4: Introduce a modern CQ creation interface")
Reported-by: Jiri Pirko <jiri@nvidia.com>
Link: https://patch.msgid.link/20260325-fix-mlx4-external-umem-v1-1-1c7c0e779329@nvidia.com
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>

drivers/infiniband/hw/mlx4/cq.c

@@ -173,6 +173,10 @@ int mlx4_ib_create_user_cq(struct ib_cq *ibcq,
 		goto err_cq;
 	}
 
+	if (ibcq->umem &&
+	    (dev->dev->caps.flags2 & MLX4_DEV_CAP_FLAG2_SW_CQ_INIT))
+		return -EOPNOTSUPP;
+
 	buf_addr = (void *)(unsigned long)ucmd.buf_addr;
 
 	if (!ibcq->umem)