RDMA/uverbs: Check existence of create_flow callback

In the accepted series "Refactor ib_uverbs_write path", we presented the
roadmap to get rid of uverbs_cmd_mask and uverbs_ex_cmd_mask fields in
favor of simple check of function pointer. So let's put NULL check of
create_flow function callback despite the fact that uverbs_ex_cmd_mask
still exists.

Link: https://www.spinics.net/lists/linux-rdma/msg60753.html
Suggested-by: Michael J Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/uverbs_cmd.c

@@ -3554,6 +3554,11 @@ int ib_uverbs_ex_create_flow(struct ib_uverbs_file *file,
 		goto err_uobj;
 	}
 
+	if (!qp->device->create_flow) {
+		err = -EOPNOTSUPP;
+		goto err_put;
+	}
+
 	flow_attr = kzalloc(struct_size(flow_attr, flows,
 				cmd.flow_attr.num_of_specs), GFP_KERNEL);
 	if (!flow_attr) {