bpf: Fix link info netfilter flags to populate defrag flag

This fix correctly populates the `bpf_link_info.netfilter.flags` field
when user passes the `BPF_F_NETFILTER_IP_DEFRAG` flag.

Fixes: 91721c2 ("netfilter: bpf: Support BPF_F_NETFILTER_IP_DEFRAG in netfilter link")
Signed-off-by: Tyrone Wu <wudevelops@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Florian Westphal <fw@strlen.de>
Cc: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/bpf/20241011193252.178997-1-wudevelops@gmail.com

net/netfilter/nf_bpf_link.c

@@ -150,11 +150,12 @@ static int bpf_nf_link_fill_link_info(const struct bpf_link *link,
 				      struct bpf_link_info *info)
 {
 	struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link);
+	const struct nf_defrag_hook *hook = nf_link->defrag_hook;
 
 	info->netfilter.pf = nf_link->hook_ops.pf;
 	info->netfilter.hooknum = nf_link->hook_ops.hooknum;
 	info->netfilter.priority = nf_link->hook_ops.priority;
-	info->netfilter.flags = 0;
+	info->netfilter.flags = hook ? BPF_F_NETFILTER_IP_DEFRAG : 0;
 
 	return 0;
 }