bpf: Reset prog callback in bpf_async_cancel_and_free()

Replace prog and callback in bpf_async_cb after removing visibility of
bpf_async_cb in bpf_async_cancel_and_free() to increase the chances the
scheduled async callbacks short-circuit execution and exit early, and
not starting a RCU tasks trace section. This improves the overall time
spent in running the wq selftest.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260205003853.527571-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: kkdwvd

kernel/bpf/helpers.c

@@ -1664,6 +1664,7 @@ static void bpf_async_cancel_and_free(struct bpf_async_kern *async)
 	if (!cb)
 		return;
 
+	bpf_async_update_prog_callback(cb, NULL, NULL);
 	/*
 	 * No refcount_inc_not_zero(&cb->refcnt) here. Dropping the last
 	 * refcnt. Either synchronously or asynchronously in irq_work.