ALSA: usx2y: Annotate struct snd_usx2y_urb_seq with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct snd_usx2y_urb_seq.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: alsa-devel@alsa-project.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230922175046.work.766-kees@kernel.orgSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/usx2y/usbusx2y.h

@@ -18,7 +18,7 @@ struct snd_usx2y_async_seq {
 struct snd_usx2y_urb_seq {
 	int	submitted;
 	int	len;
-	struct urb	*urb[];
+	struct urb	*urb[] __counted_by(len);
 };
 
 #include "usx2yhwdeppcm.h"

sound/usb/usx2y/usbusx2yaudio.c

@@ -681,6 +681,7 @@ static int usx2y_rate_set(struct usx2ydev *usx2y, int rate)
 			err = -ENOMEM;
 			goto cleanup;
 		}
+		us->len = NOOF_SETRATE_URBS;
 		usbdata = kmalloc_array(NOOF_SETRATE_URBS, sizeof(int),
 					GFP_KERNEL);
 		if (!usbdata) {
@@ -702,7 +703,6 @@ static int usx2y_rate_set(struct usx2ydev *usx2y, int rate)
 		if (err < 0)
 			goto cleanup;
 		us->submitted =	0;
-		us->len =	NOOF_SETRATE_URBS;
 		usx2y->us04 =	us;
 		wait_event_timeout(usx2y->in04_wait_queue, !us->len, HZ);
 		usx2y->us04 =	NULL;