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.