selftests/bpf: Fix resource leak in serial_test_wq on attach failure

selftests/bpf: Fix resource leak in serial_test_wq on attach failure

When wq__attach() fails, serial_test_wq() returns early without calling
wq__destroy(), leaking the skeleton resources allocated by
wq__open_and_load(). This causes ASAN leak reports in selftests runs.

Fix this by jumping to a common clean_up label that calls wq__destroy()
on all exit paths after successful open_and_load.

Note that the early return after wq__open_and_load() failure is correct
and doesn't need fixing, since that function returns NULL on failure
(after internally cleaning up any partial allocations).

Fixes: 8290dba51910 ("selftests/bpf: wq: add bpf_wq_start() checks")
Signed-off-by: Kery Qi <qikeyu2017@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20260121094114.1801-3-qikeyu2017@gmail.com