sched: Be more strict about p->is_blocked

sched: Be more strict about p->is_blocked

Upon entry to try_to_block_task(), p->is_blocked should be false. After all,
the prior wakeup would have made it so per ttwu_do_wakeup().

Ensure this is the case, rather than clearing it in the path that doesn't set
it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: John Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260526113322.364017314%40infradead.org