sched: Add is_blocked task flag

sched: Add is_blocked task flag

Add a new is_blocked flag to the task struct. This flag is set
by try_to_block_task() and cleared by ttwu_do_wakeup() and
tracks if the task is blocked.

Traditionally this would mirror !p->on_rq, however due things
like DELAY_DEQUEUE and PROXY_EXEC, this can diverge, so its
useful to manage separately.

Additionally with this, we might be able to get rid of the
p->se.sched_delayed (ab)use in the core code (eventually).

Taken whole cloth from Peter's email:
  https://lore.kernel.org/lkml/20260501132143.GC1026330@noisy.programming.kicks-ass.net/

With a few additional p->is_blocked = 0 in a few cases where
we return current if blocked_on gets zeroed or there is
no owner. This may hint that these current special cases
might be dropped eventually.

This change also helps resolve wait-queue stalls seen with
proxy-execution. See previous patch attempts for details:
  https://lore.kernel.org/lkml/20260430215103.2978955-2-jstultz@google.com/

Reported-by: Vineeth Pillai <vineethrp@google.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260512025635.2840817-7-jstultz@google.com