ALSA: core: Serialize deferred fasync state checks

ALSA: core: Serialize deferred fasync state checks

snd_fasync_helper() updates fasync->on under snd_fasync_lock, and
snd_fasync_work_fn() now also evaluates fasync->on under the same
lock. snd_kill_fasync() still tests the flag before taking the lock,
leaving an unsynchronized read against FASYNC enable/disable updates.

Move the enabled-state check into the locked section.

Also clear fasync->on under snd_fasync_lock in snd_fasync_free()
before unlinking the pending entry. Together with the locked sender-side
check, this publishes teardown before flushing the deferred work and
prevents a racing sender from requeueing the entry after free has
started.

Fixes: ef34a0ae7a26 ("ALSA: core: Add async signal helpers")
Fixes: 8146cd333d23 ("ALSA: core: Fix potential data race at fasync handling")
Cc: stable@vger.kernel.org
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260506-alsa-core-fasync-on-lock-v1-1-ea48c77d6ca4@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>