bpf: Enable bpf_timer and bpf_wq in any context

Refactor bpf_timer and bpf_wq to allow calling them from any context:
- add refcnt to bpf_async_cb
- map_delete_elem or map_free will drop refcnt to zero
  via bpf_async_cancel_and_free()
- once refcnt is zero timer/wq_start is not allowed to make sure
  that callback cannot rearm itself
- if in_hardirq defer to start/cancel operations to irq_work

Co-developed-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/20260201025403.66625-2-alexei.starovoitov@gmail.com

Author: Alexei Starovoitov