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

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].