sound/virtio: Fix cancel_sync warnings on uninitialized work_structs

Betty reported hitting the following warning:

[    8.709131][  T221] WARNING: CPU: 2 PID: 221 at kernel/workqueue.c:4182
...
[    8.713282][  T221] Call trace:
[    8.713365][  T221]  __flush_work+0x8d0/0x914
[    8.713468][  T221]  __cancel_work_sync+0xac/0xfc
[    8.713570][  T221]  cancel_work_sync+0x24/0x34
[    8.713667][  T221]  virtsnd_remove+0xa8/0xf8 [virtio_snd ab15f34d0dd772f6d11327e08a81d46dc9c36276]
[    8.713868][  T221]  virtsnd_probe+0x48c/0x664 [virtio_snd ab15f34d0dd772f6d11327e08a81d46dc9c36276]
[    8.714035][  T221]  virtio_dev_probe+0x28c/0x390
[    8.714139][  T221]  really_probe+0x1bc/0x4c8
...

It seems we're hitting the error path in virtsnd_probe(), which
triggers a virtsnd_remove() which iterates over the substreams
calling cancel_work_sync() on the elapsed_period work_struct.

Looking at the code, from earlier in:
virtsnd_probe()->virtsnd_build_devs()->virtsnd_pcm_parse_cfg()

We set snd->nsubstreams, allocate the snd->substreams, and if
we then hit an error on the info allocation or something in
virtsnd_ctl_query_info() fails, we will exit without having
initialized the elapsed_period work_struct.

When that error path unwinds we then call virtsnd_remove()
which as long as the substreams array is allocated, will iterate
through calling cancel_work_sync() on the uninitialized work
struct hitting this warning.

Takashi Iwai suggested this fix, which initializes the substreams
structure right after allocation, so that if we hit the error
paths we avoid trying to cleanup uninitialized data.

Note: I have not yet managed to reproduce the issue myself, so
this patch has had limited testing.

Feedback or thoughts would be appreciated!

Cc: Anton Yakovlev <anton.yakovlev@opensynergy.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: virtualization@lists.linux.dev
Cc: linux-sound@vger.kernel.org
Cc: kernel-team@android.com
Reported-by: Betty Zhou <bettyzhou@google.com>
Suggested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: John Stultz <jstultz@google.com>
Message-Id: <20250116194114.3375616-1-jstultz@google.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>