bpf: Introduce lock-free bpf_async_update_prog_callback()

Introduce bpf_async_update_prog_callback(): lock-free update of cb->prog
and cb->callback_fn. This function allows updating prog and callback_fn
fields of the struct bpf_async_cb without holding lock.
For now use it under the lock from __bpf_async_set_callback(), in the
next patches that lock will be removed.

Lock-free algorithm:
 * Acquire a guard reference on prog to prevent it from being freed
   during the retry loop.
 * Retry loop:
    1. Each iteration acquires a new prog reference and stores it
       in cb->prog via xchg. The previous prog is released.
    2. The loop condition checks if both cb->prog and cb->callback_fn
       match what we just wrote. If either differs, a concurrent writer
       overwrote our value, and we must retry.
    3. When we retry, our previously-stored prog was already released by
       the concurrent writer or will be released by us after
       overwriting.
 * Release guard reference.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260120-timer_nolock-v6-3-670ffdd787b4@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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