auxdisplay: line-display: fix NULL dereference in linedisp_release

In the Linux kernel, the following vulnerability has been resolved:

auxdisplay: line-display: fix NULL dereference in linedisp_release

linedisp_release() currently retrieves the enclosing struct linedisp via
to_linedisp(). That lookup depends on the attachment list, but the
attachment may already have been removed before put_device() invokes the
release callback. This can happen in linedisp_unregister(), and can also
be reached from some linedisp_register() error paths.

In that case, to_linedisp() returns NULL and linedisp_release()
dereferences it while freeing the display resources.

The struct device released here is the embedded linedisp->dev used by
linedisp_register(), so retrieve the enclosing object directly with
container_of() instead.