iio: buffer: Fix potential use-after-free in anonymous buffer release

Summary

In the Linux kernel, the following vulnerability has been resolved: iio: buffer: Fix potential use-after-free in anonymous buffer release An anonymous buffer handle holds a reference to the underlying IIO device. The reference is dropped in the buffer handle's release function. If the device has been removed, either through unbind or hot-unplug, the buffer handle might hold the last reference. The release function takes the mutex for the buffer using a guard, which means the unlock happens after all the code in the function, including `iio_device_put()`. If the anonymous buffer holds the last reference this might free both the IIO device and the buffer, which contains the mutex, leading to use-after-free when the mutex is unlocked. Fix this by using a scoped guard just around the buffer dmabuf list access, making sure the mutex is unlocked before releasing the IIO device. Version 10 of the patch that introduced this issue used this exact scheme of first unlocking and then dropping the reference [1]. During review it was suggested to use a guard instead, and version 11 made that change [2].