usb: ch9: Replace bmSublinkSpeedAttr 1-element array with flexible array

Since commit 2d47c695 ("ubsan: Tighten UBSAN_BOUNDS on GCC"),
UBSAN_BOUNDS no longer pretends 1-element arrays are unbounded. Walking
bmSublinkSpeedAttr will trigger a warning, so make it a proper flexible
array. Add a union to keep the struct size identical for userspace in
case anything was depending on the old size.

False positive warning was:

UBSAN: array-index-out-of-bounds in drivers/usb/host/xhci-hub.c:231:31 index 1 is out of range for type '__le32 [1]'

for this line of code:

	ssp_cap->bmSublinkSpeedAttr[offset++] = cpu_to_le32(attr);
Reported-by: Borislav Petkov <bp@alien8.de>
Closes: https://lore.kernel.org/lkml/2023062945-fencing-pebble-0411@gregkh/Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/lkml/9a8e34ad-8a8b-3830-4878-3c2c82e69dd9@alu.unizg.hr/
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Tested-by: "Borislav Petkov (AMD)" <bp@alien8.de>
Tested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230629190900.never.787-kees@kernel.orgSigned-off-by: Kees Cook <keescook@chromium.org>

include/uapi/linux/usb/ch9.h

@@ -981,7 +981,11 @@ struct usb_ssp_cap_descriptor {
 #define USB_SSP_MIN_RX_LANE_COUNT		(0xf << 8)
 #define USB_SSP_MIN_TX_LANE_COUNT		(0xf << 12)
 	__le16 wReserved;
-	__le32 bmSublinkSpeedAttr[1]; /* list of sublink speed attrib entries */
+	union {
+		__le32 legacy_padding;
+		/* list of sublink speed attrib entries */
+		__DECLARE_FLEX_ARRAY(__le32, bmSublinkSpeedAttr);
+	};
 #define USB_SSP_SUBLINK_SPEED_SSID	(0xf)		/* sublink speed ID */
 #define USB_SSP_SUBLINK_SPEED_LSE	(0x3 << 4)	/* Lanespeed exponent */
 #define USB_SSP_SUBLINK_SPEED_LSE_BPS		0