bpf: wq: add bpf_wq_set_callback_impl

To support sleepable async callbacks, we need to tell push_async_cb()
whether the cb is sleepable or not.

The verifier now detects that we are in bpf_wq_set_callback_impl and
can allow a sleepable callback to happen.

Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
Link: https://lore.kernel.org/r/20240420-bpf_wq-v2-13-6c986a5a741f@kernel.org


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