ALSA: aloop: Don't constrain playback params in notify mode

With notify enabled (the pcm_notify module parameter or the "PCM
Notify" control), the playback side of a cable may change format, rate
and channels while a capture is running. loopback_check_format() then
stops the capture and notifies the "PCM Slave" controls, so that the
capture application can reopen with the new parameters. For this
reason loopback_open() still gives a notify mode playback the
unconstrained loopback_pcm_hardware.

Since commit 898dfe4687f4 ("ALSA: aloop: Fix racy hw constraints
adjustment") the hw rules refine against the shared cable->hw instead
of the substream's own runtime->hw. This pins the playback side to the
parameters of an open capture whatever the notify setting is. A player
probing the device sees only the capture's rate, format and channels,
and the format change path in loopback_check_format() can't be reached
by a new playback.

Skip the format, rate and channels rules for the playback substream
when notify is enabled. Skip the period bytes rule as well: with a
sound timer source, params_change() already takes the period size from
a notify mode playback, and keeping the old capture's period bytes
leaves a playback at another rate with a period that doesn't match the
timer. The capture side stays constrained to the cable parameters.

The capture stop this makes reachable again was hardened by commit
826af7fa62e3 ("ALSA: aloop: Fix racy access at PCM trigger") and
commit e5c33cdc6f40 ("ALSA: aloop: Fix peer runtime UAF during
format-change stop"), which need to be applied first when backporting.

Reproducer, with notify enabled on the cable:
  arecord -D hw:Loopback,1,0 -r 44100 -f S32_LE -c 2 /dev/null &
  aplay -D hw:Loopback,0,0 96k.wav
Before: aplay is forced to 44100 Hz and the capture keeps running.
After: aplay runs at 96000 Hz, the capture is stopped and "PCM Slave
Rate" reports 96000.

Fixes: 898dfe4687f4 ("ALSA: aloop: Fix racy hw constraints adjustment")
Reported-by: Pavel Hofman <pavel.hofman@ivitera.com>
Closes: https://lore.kernel.org/all/b4af9071-f8d7-5b47-4d7a-c5743bd67394@ivitera.com/
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Henrik Enquist <henrik.enquist@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20261003-aloop-notify-fix-v1-1-ba5dfb831bf5@gmail.com

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