selftests/bpf: add bpf task work stress tests

Add stress tests for BPF task-work scheduling kfuncs. The tests spawn
multiple threads that concurrently schedule task_work callbacks against
the same and different map values to exercise the kfuncs under high
contention.
Verify callbacks are reliably enqueued and executed with no drops.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20250923112404.668720-10-mykyta.yatsenko5@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>