usb: gadget: uvc: use correct buffer size when parsing configfs lists

In the Linux kernel, the following vulnerability has been resolved:

usb: gadget: uvc: use correct buffer size when parsing configfs lists

This commit fixes uvc gadget support on 32-bit platforms.

Commit 0df28607c5cb ("usb: gadget: uvc: Generalise helper functions for

reuse") introduced a helper function __uvcg_iter_item_entries() to aid

with parsing lists of items on configfs attributes stores. This function

is a generalization of another very similar function, which used a

stack-allocated temporary buffer of fixed size for each item in the list

and used the sizeof() operator to check for potential buffer overruns.

The new function was changed to allocate the now variably sized temp

buffer on heap, but wasn't properly updated to also check for max buffer

size using the computed size instead of sizeof() operator.

As a result, the maximum item size was 7 (plus null terminator) on

64-bit platforms, and 3 on 32-bit ones. While 7 is accidentally just

barely enough, 3 is definitely too small for some of UVC configfs

attributes. For example, dwFrameInteval, specified in 100ns units,

usually has 6-digit item values, e.g. 166666 for 60fps.