bpf: bpf_prog_array_copy() should return -ENOENT if exclude_prog not found

This makes is it possible for bpf prog detach to return -ENOENT.

Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>