samples/bpf: Check detach prog exist or not in xdp_fwd

Before detach the prog, we should check detach prog exist or not.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20220606005425.261967-1-shaozhengchao@huawei.com