scx_p2dq: update ATQ API to the newest version

[etsal]

Update the p2dq scheduler to use the new ATQ API:

1. Add task_ctx_common to the scheduler's task_p2dq task state.
2. Update all scx_atq_*() call sites to use the embedded task_ctx_common struct instead of pushing/popping the PID of the task. Remove the DSQ fallback path now that the ATQ has infinite size.
3. Remove the task lookup call sites, since we're now grabbing the task_ctx directly from the ATQ. Add the PID to task_p2dq to enable task_ctx -> struct task_struct lookups.
4. Factor out the ATQ membership cancelling code into atq.bpf.c from cgroup_bw.bpf.c, and use it to implement .dequeue() for p2dq.

cc: Daniel Hodges hodgesds@meta.com
Signed-off-by: Emil Tsalapatis emil@etsalapatis.com

[etsal]

cc @hodgesds

[multics69]

Thanks, @etsal ! LGTM