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: 89ae89f5 ("bpf: Add multi uprobe link")
Signed-off-by: Tao Chen <chen.dylane@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250407035752.1108927-2-chen.dylane@linux.dev