iio: buffer: serialize buffer teardown with mode claims

In the Linux kernel, the following vulnerability has been resolved: iio: buffer: serialize buffer teardown with mode claims Buffer-mode claims hold mlock to guarantee that the device remains in buffer mode until the claim is released. Normal buffer updates take info_exist_lock followed by mlock in iio_update_buffers(). However, iio_device_unregister() disables and deactivates all buffers without taking mlock. This can invalidate buffer state, including active_scan_mask, while a buffer-mode claim is held. Take mlock in iio_disable_all_buffers() so that unregister honors the mode-claim lifetime guarantee. The info_exist_lock -> mlock ordering matches iio_update_buffers().