gpio: sloppy-logic-analyzer: fix use-after-free via debugfs trigger on unbind

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

gpio: sloppy-logic-analyzer: fix use-after-free via debugfs trigger on unbind

The "trigger" debugfs file has a hand-rolled ->write handler
(trigger_write()) that dereferences the per-device gpio_la_poll_priv. The
file is created with debugfs_create_file_unsafe(), and the handler never
takes a debugfs reference. Nothing keeps the object alive while the
handler runs.

priv is allocated with devm_kzalloc(). devres frees it when the platform
device is unbound. debugfs_create_file_unsafe() installs no full_proxy
wrapper, so debugfs_remove_recursive() in gpio_la_poll_remove() does not
wait for an in-flight trigger_write(). The blob_lock taken there does not
help, because trigger_write() never takes it. A write that races an unbind
therefore writes into freed memory:

trigger_write() gpio_la_poll_remove()
priv = m->private
buf = memdup_user() [may sleep]
mutex_lock(&priv->blob_lock)
debugfs_remove_recursive() [no wait]
mutex_unlock(&priv->blob_lock)
(remove returns; devres frees priv)
priv->trig_data = buf <-- use-after-free write
priv->trig_len = count

The race is reachable by root via
/sys/bus/platform/drivers/gpio-sloppy-logic-analyzer/unbind.

Create "trigger" with debugfs_create_file() instead. Its full_proxy
wrapper makes debugfs_remove_recursive() drain any in-flight ->write
before it returns.

The use-after-free is confirmed under KASAN with a minimal reproducer of
the same debugfs_create_file_unsafe() plus devm_kzalloc() pattern
(available on request); it produces a slab-use-after-free write in the
handler.