bpf: task work scheduling kfuncs

Implementation of the new bpf_task_work_schedule kfuncs, that let a BPF
program schedule task_work callbacks for a target task:
 * bpf_task_work_schedule_signal() - schedules with TWA_SIGNAL
 * bpf_task_work_schedule_resume() - schedules with TWA_RESUME

Each map value should embed a struct bpf_task_work, which the kernel
side pairs with struct bpf_task_work_kern, containing a pointer to
struct bpf_task_work_ctx, that maintains metadata relevant for the
concrete callback scheduling.

A small state machine and refcounting scheme ensures safe reuse and
teardown. State transitions:
    _______________________________
    |                             |
    v                             |
[standby] ---> [pending] --> [scheduling] --> [scheduled]
    ^                             |________________|_________
    |                                                       |
    |                                                       v
    |                                                   [running]
    |_______________________________________________________|

All states may transition into FREED state:
[pending] [scheduling] [scheduled] [running] [standby] -> [freed]

A FREED terminal state coordinates with map-value
deletion (bpf_task_work_cancel_and_free()).

Scheduling itself is deferred via irq_work to keep the kfunc callable
from NMI context.

Lifetime is guarded with refcount_t + RCU Tasks Trace.

Main components:
 * struct bpf_task_work_context – Metadata and state management per task
work.
 * enum bpf_task_work_state – A state machine to serialize work
 scheduling and execution.
 * bpf_task_work_schedule() – The central helper that initiates
scheduling.
 * bpf_task_work_acquire_ctx() - Attempts to take ownership of the context,
 pointed by passed struct bpf_task_work, allocates new context if none
 exists yet.
 * bpf_task_work_callback() – Invoked when the actual task_work runs.
 * bpf_task_work_irq() – An intermediate step (runs in softirq context)
to enqueue task work.
 * bpf_task_work_cancel_and_free() – Cleanup for deleted BPF map entries.

Flow of successful task work scheduling
 1) bpf_task_work_schedule_* is called from BPF code.
 2) Transition state from STANDBY to PENDING, mark context as owned by
 this task work scheduler
 3) irq_work_queue() schedules bpf_task_work_irq().
 4) Transition state from PENDING to SCHEDULING (noop if transition
 successful)
 5) bpf_task_work_irq() attempts task_work_add(). If successful, state
 transitions to SCHEDULED.
 6) Task work calls bpf_task_work_callback(), which transition state to
 RUNNING.
 7) BPF callback is executed
 8) Context is cleaned up, refcounts released, context state set back to
 STANDBY.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Reviewed-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250923112404.668720-8-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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