ALSA: usx2y: Prefer struct_size over open coded arithmetic

As noted in the "Deprecated Interfaces, Language Features, Attributes,
and Conventions" documentation [1], size calculations (especially
multiplication) should not be performed in memory allocator (or similar)
function arguments due to the risk of them overflowing. This could lead
to values wrapping around and a smaller allocation being made than the
caller was expecting. Using those allocations could lead to linear
overflows of heap memory and other misbehaviors.

In this case this is not actually dynamic size: all the operands
involved in the calculation are constant values. However it is better to
refactor this anyway, just to keep the open-coded math idiom out of
code.

So, use the struct_size() helper to do the arithmetic instead of the
argument "size + size * count" in the kzalloc() function.

Also, take the opportunity to refactor the declaration variables to make
it more easy to read.

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-argumentsSigned-off-by: Len Baker <len.baker@gmx.com>
Link: https://lore.kernel.org/r/20210919133727.44694-1-len.baker@gmx.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/usx2y/usbusx2yaudio.c

@@ -668,14 +668,15 @@ static void i_usx2y_04int(struct urb *urb)
 
 static int usx2y_rate_set(struct usx2ydev *usx2y, int rate)
 {
-	int			err = 0, i;
-	struct snd_usx2y_urb_seq	*us = NULL;
-	int			*usbdata = NULL;
-	const struct s_c2	*ra = rate == 48000 ? setrate_48000 : setrate_44100;
+	int err = 0, i;
+	struct snd_usx2y_urb_seq *us = NULL;
+	int *usbdata = NULL;
+	const struct s_c2 *ra = rate == 48000 ? setrate_48000 : setrate_44100;
 	struct urb *urb;
 
 	if (usx2y->rate != rate) {
-		us = kzalloc(sizeof(*us) + sizeof(struct urb *) * NOOF_SETRATE_URBS, GFP_KERNEL);
+		us = kzalloc(struct_size(us, urb, NOOF_SETRATE_URBS),
+			     GFP_KERNEL);
 		if (!us) {
 			err = -ENOMEM;
 			goto cleanup;