selftests/bpf: Task_work selftest cleanup fixes

task_work selftest does not properly handle cleanup during failures:
 * destroy bpf_link
 * perf event fd is passed to bpf_link, no need to close it if link was
 created successfully
 * goto cleanup if fork() failed, close pipe.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250924142954.129519-2-mykyta.yatsenko5@gmail.com