iio: core: move @info_exist_lock to struct iio_dev_opaque

This lock is only of interest to the IIO core, so make it only
visible there.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210426174911.397061-7-jic23@kernel.org