io_uring: ensure io_queue_deferred() is out-of-line

This is not the hot path, it's a slow path. Yet the locking for it is
in the hot path, and __cold does not prevent it from being inlined.

Move the locking to the function itself, and mark it noinline as well
to avoid it polluting the icache of the hot path.

Signed-off-by: Jens Axboe <axboe@kernel.dk>