sched/proxy: Switch proxy to use p->is_blocked

sched/proxy: Switch proxy to use p->is_blocked

Rather than gate the proxy paths with p->blocked_on, use p->is_blocked.

This opens up the state: '->is_blocked && !->blocked_on' for future use.

Notably, only proxy and delayed tasks can be ->on_rq && ->is_blocked, and it is
guaranteed that sched_class::pick_task() will never return a delayed task.
Therefore any task returned from pick_next_task() that has ->is_blocked set,
must be a proxy task.

Suggested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260526113322.477954312%40infradead.org