usb: gadget: f_fs: stricter integer overflow checks

This from static analysis.  The vla_item() takes a size and adds it to
the total.  It has a built in integer overflow check so if it encounters
an integer overflow anywhere then it records the total as SIZE_MAX.

However there is an issue here because the "lang_count*(needed_count+1)"
multiplication can overflow.  Technically the "lang_count + 1" addition
could overflow too, but that would be detected and is harmless.  Fix
both using the new size_add() and size_mul() functions.

Fixes: e6f3862f ("usb: gadget: FunctionFS: Remove VLAIS usage from gadget code")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YxDI3lMYomE7WCjn@kiliSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_fs.c

@@ -2645,10 +2645,10 @@ static int __ffs_data_got_strings(struct ffs_data *ffs,
 		unsigned i = 0;
 		vla_group(d);
 		vla_item(d, struct usb_gadget_strings *, stringtabs,
-			lang_count + 1);
+			size_add(lang_count, 1));
 		vla_item(d, struct usb_gadget_strings, stringtab, lang_count);
 		vla_item(d, struct usb_string, strings,
-			lang_count*(needed_count+1));
+			size_mul(lang_count, (needed_count + 1)));
 
 		char *vlabuf = kmalloc(vla_group_size(d), GFP_KERNEL);