selftests/bpf: Fix leaked bpf_link in get_stackid_cannot_attach

skel->links.oncpu is leaked in one case. This causes test perf_branches
fails when it runs after get_stackid_cannot_attach:

./test_progs -t get_stackid_cannot_attach,perf_branches
84      get_stackid_cannot_attach:OK
test_perf_branches_common:PASS:test_perf_branches_load 0 nsec
test_perf_branches_common:PASS:attach_perf_event 0 nsec
test_perf_branches_common:PASS:set_affinity 0 nsec
check_good_sample:FAIL:output not valid no valid sample from prog
146/1   perf_branches/perf_branches_hw:FAIL
146/2   perf_branches/perf_branches_no_hw:OK
146     perf_branches:FAIL

All error logs:
test_perf_branches_common:PASS:test_perf_branches_load 0 nsec
test_perf_branches_common:PASS:attach_perf_event 0 nsec
test_perf_branches_common:PASS:set_affinity 0 nsec
check_good_sample:FAIL:output not valid no valid sample from prog
146/1   perf_branches/perf_branches_hw:FAIL
146     perf_branches:FAIL
Summary: 1/1 PASSED, 0 SKIPPED, 1 FAILED

Fix this by adding the missing bpf_link__destroy().

Fixes: 346938e9

 ("selftests/bpf: Add get_stackid_cannot_attach")
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230412210423.900851-3-song@kernel.org