ALSA: virtio: Replace zero-length array with flexible-array member

There is a regular need in the kernel to provide a way to declare
having a dynamically sized set of trailing elements in a structure.
Kernel code should always use “flexible array members”[1] for these
cases. The older style of one-element or zero-length arrays should
no longer be used[2].

Also, make use of the struct_size() helper in kzalloc().

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/78Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210929191504.GA337268@embeddedorSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

sound/virtio/virtio_pcm_msg.c

@@ -20,7 +20,7 @@ struct virtio_pcm_msg {
 	struct virtio_snd_pcm_xfer xfer;
 	struct virtio_snd_pcm_status status;
 	size_t length;
-	struct scatterlist sgs[0];
+	struct scatterlist sgs[];
 };
 
 /**
@@ -146,8 +146,7 @@ int virtsnd_pcm_msg_alloc(struct virtio_pcm_substream *vss,
 		int sg_num = virtsnd_pcm_sg_num(data, period_bytes);
 		struct virtio_pcm_msg *msg;
 
-		msg = kzalloc(sizeof(*msg) + sizeof(*msg->sgs) * (sg_num + 2),
-			      GFP_KERNEL);
+		msg = kzalloc(struct_size(msg, sgs, sg_num + 2), GFP_KERNEL);
 		if (!msg)
 			return -ENOMEM;