selftests/bpf: Destroy the skeleton when CONFIG_PREEMPT is off

Destroy the created skeleton when CONFIG_PREEMPT is off, else will be
resource leak.

Fixes: 73b97bc7 ("selftests/bpf: Test concurrent updates on bpf_task_storage_busy")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20220921070035.2016413-2-houtao@huaweicloud.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/map_tests/task_storage_map.c

@@ -79,6 +79,7 @@ void test_task_storage_map_stress_lookup(void)
 	/* Only for a fully preemptible kernel */
 	if (!skel->kconfig->CONFIG_PREEMPT) {
 		printf("%s SKIP (no CONFIG_PREEMPT)\n", __func__);
+		read_bpf_task_storage_busy__destroy(skel);
 		skips++;
 		return;
 	}