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.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>