ALSA: seq: Don't leak the extension cell pointer in the bounce payload

The bounce_error_event() embeds the failed event in the bounce payload
by pointing data.ext.ptr at it.  When that event is a queued
variable-length event, its own data.ext.ptr holds the address of its
first extension cell, put there by snd_seq_event_dup().  The payload
goes out verbatim through snd_seq_expand_var_event(), so the address
reaches userspace.

That is the same address commit 705dd6dcbc0e ("ALSA: seq: Clear
variable event pointer on read") removed from the event header.  The
read path still clears it there, just above the call that expands the
payload.

Embed a sanitised copy instead, treated exactly as snd_seq_read()
treats the header.  A stack copy is enough because delivery is
synchronous and snd_seq_event_dup() copies before returning.

An unprivileged client reaches this by setting SNDRV_SEQ_FILTER_BOUNCE,
queueing a variable-length event to a port that does not exist and
reading the bounce back.  Eight bytes on 64-bit, from its own pool.

Fixes: efc86691e4d8 ("ALSA: seq: Fix kernel heap address leak in bounce_error_event()")
Assisted-by: Claude:claude-opus-5
Signed-off-by: HyeongJun An <sammiee5311@gmail.com>
Link: https://patch.msgid.link/20260811131835.3837024-1-sammiee5311@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>