bpftool: Change pid_iter.bpf.c to comply with the change of bpf_link_fops.

To support epoll, a new instance of file_operations, bpf_link_fops_poll,
has been added for links that support epoll. The pid_iter.bpf.c checks
f_ops for links and other BPF objects. The check should fail for struct_ops
links without this patch.

Acked-by: Quentin Monnet <[e-mail]>
Signed-off-by: Kui-Feng Lee <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

tools/bpf/bpftool/skeleton/pid_iter.bpf.c

···
 };
 
 extern const void bpf_link_fops __ksym;
+extern const void bpf_link_fops_poll __ksym __weak;
 extern const void bpf_map_fops __ksym;
 extern const void bpf_prog_fops __ksym;
 extern const void btf_fops __ksym;
···
 		fops = &btf_fops;
 		break;
 	case BPF_OBJ_LINK:
-		fops = &bpf_link_fops;
+		if (&bpf_link_fops_poll &&
+		    file->f_op == &bpf_link_fops_poll)
+			fops = &bpf_link_fops_poll;
+		else
+			fops = &bpf_link_fops;
 		break;
 	default:
 		return 0;