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: 91721c2d02d3 ("netfilter: bpf: Support BPF_F_NETFILTER_IP_DEFRAG in netfilter link")
Signed-off-by: Tyrone Wu <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Florian Westphal <[e-mail]>
Cc: Daniel Xu <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

net/netfilter/nf_bpf_link.c

···
 				      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;
 }