iio: consumers: ensure read buffers for labels and ext_info are page aligned

Attributes of iio providers are exposed via sysfs. Typically, providers
pass attribute values to the iio core, which handles formatting and
printing to sysfs. However, some attributes, such as labels or extended
info, are directly formatted and printed to sysfs by provider drivers
using sysfs_emit() and sysfs_emit_at(). These helpers assume the read
buffer, allocated by sysfs fop, is page-aligned. When these attributes
are accessed by consumer drivers, the read buffer is allocated by the
consumer and may not be page-aligned, leading to failures in the
provider's callback that utilizes sysfs_emit*.

Add a check to ensure that read buffers for labels and external info
attributes are page-aligned. Update the prototype documentation as well.

Signed-off-by: Matteo Martelli <matteomartelli3@gmail.com>
Link: https://patch.msgid.link/20241202-iio-kmalloc-align-v1-1-aa9568c03937@gmail.com


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