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

tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c

@@ -204,8 +204,8 @@ static int pass_ack(struct migrate_reuseport_test_case *test_case)
 {
 	int err;
 
-	err = bpf_link__detach(test_case->link);
-	if (!ASSERT_OK(err, "bpf_link__detach"))
+	err = bpf_link__destroy(test_case->link);
+	if (!ASSERT_OK(err, "bpf_link__destroy"))
 		return -1;
 
 	test_case->link = NULL;