sched: Improve try_invoke_on_locked_down_task()

Clarify and tighten try_invoke_on_locked_down_task().

Basically the function calls @func under task_rq_lock(), except it
avoids taking rq->lock when possible.

This makes calling @func unconditional (the function will get renamed
in a later patch to remove the try).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Tested-by: Vasily Gorbik <gor@linux.ibm.com> # on s390
Link: https://lkml.kernel.org/r/20210929152428.589323576@infradead.org