block: ublk_drv: clean up several helpers

Convert the following pattern in several helpers
	if (Z)
		return true
	return false
into:
	return Z;

Reviewed-by: Ziyang Zhang <ZiyangZhang@linux.alibaba.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>