selftests/bpf: Destroy XDP link correctly

bpf_link__detach() was confused with bpf_link__destroy() and leaves
leaked FD in the process. Fix the problem.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Hengqi Chen <hengqi.chen@gmail.com>
Link: https://lore.kernel.org/bpf/20211107165521.9240-9-andrii@kernel.org