iio: iio-mux: kzalloc instead of devm_kzalloc to ensure page alignment

iio: iio-mux: kzalloc instead of devm_kzalloc to ensure page alignment

During channel configuration, the iio-mux driver allocates a page with
devm_kzalloc(PAGE_SIZE) to read channel ext_info. However, the resulting
buffer points to an offset of the page due to the devres header sitting
at the beginning of the allocated area. This leads to failure in the
provider driver when sysfs_emit* helpers are used to format the ext_info
attributes.

Switch to plain kzalloc version. The devres version is not strictly
necessary as the buffer is only accessed during the channel
configuration phase. Rely on __free cleanup to deallocate the buffer.
Also, move the ext_info handling into a new function to have the page
buffer definition and assignment in one statement as suggested by
cleanup documentation.

Signed-off-by: Matteo Martelli <matteomartelli3@gmail.com>
Fixes: 7ba9df54b091 ("iio: multiplexer: new iio category and iio-mux driver")
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241202-iio-kmalloc-align-v1-2-aa9568c03937@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>