iio: kfifo-buffer: Add output buffer support

Add output buffer support to the kfifo buffer implementation.

The implementation is straight forward and mostly just wraps the kfifo
API to provide the required operations.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Mihail Chindris <mihail.chindris@analog.com>
Link: https://lore.kernel.org/r/20211007080035.2531-3-mihail.chindris@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>