bpf: Check link_create.flags parameter for multi_uprobe

The link_create.flags are currently not used for multi-uprobes, so return
-EINVAL if it is set, same as for other attach APIs.

We allow target_fd to have an arbitrary value for multi-uprobe, though,
as there are existing users (libbpf) relying on this.

Fixes: 89ae89f53d20 ("bpf: Add multi uprobe link")
Signed-off-by: Tao Chen <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/trace/bpf_trace.c

···
 	if (sizeof(u64) != sizeof(void *))
 		return -EOPNOTSUPP;
 
+	if (attr->link_create.flags)
+		return -EINVAL;
+
 	if (!is_uprobe_multi(prog))
 		return -EINVAL;