ALSA: pcm: Protect the unregister notifier traversal

snd_pcm_free() traverses snd_pcm_notify_list and calls n_unregister
without register_mutex, although snd_pcm_notify() takes that mutex to
add and remove notifiers. PCM disconnect releases the mutex before the
free callback runs, so it does not protect this later traversal.

With no open OSS file, normal snd-pcm-oss removal can overlap PCM card
teardown, for example when unbinding snd-dummy:

  PCM card teardown                     snd-pcm-oss unload
  -----------------                     ------------------
  snd_pcm_dev_disconnect()
    remove PCM under register_mutex
    unlock register_mutex
  snd_pcm_free()
    fetch notifier
                                        snd_pcm_notify(..., 1)
                                          unlink notifier under mutex
                                        free module storage
    read notifier->n_unregister
    invoke callback

The free path can then read the notifier from released module storage
or call its unloaded code.

Hold register_mutex across the unregister traversal and its callbacks.
Notifier removal must wait until both have finished. This also protects
the traversal if a callback releases the last module reference: module
exit cannot unlink its notifier until the free path releases the mutex.
Drop the mutex before private_free and the remaining stream cleanup.

Fixes: c461482c8072 ("[ALSA] Unregister device files at disconnection")
Assisted-by: LLM
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/pm-pmb-26d66390a25fec9fad6c-v6-5be251a3ac8832955a31-1@gmail.com

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