iio: buffer: Expose data available

Add a sysfs attribute that exposes buffer data available to userspace.
This attribute can be checked at runtime to determine the overall buffer
fill level (across all allocated buffers).

Signed-off-by: Matt Fornero <matt.fornero@mathworks.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>