Input: omap-keypad - 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.

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

[1] https://www.kernel.org/doc/html/v5.14/process/deprecated.html#open-coded-arithmetic-in-allocator-argumentsSigned-off-by: Len Baker <len.baker@gmx.com>
Link: https://lore.kernel.org/r/20210911112716.10067-1-len.baker@gmx.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/keyboard/omap-keypad.c

@@ -190,8 +190,7 @@ static int omap_kp_probe(struct platform_device *pdev)
 	row_shift = get_count_order(pdata->cols);
 	keycodemax = pdata->rows << row_shift;
 
-	omap_kp = kzalloc(sizeof(struct omap_kp) +
-			keycodemax * sizeof(unsigned short), GFP_KERNEL);
+	omap_kp = kzalloc(struct_size(omap_kp, keymap, keycodemax), GFP_KERNEL);
 	input_dev = input_allocate_device();
 	if (!omap_kp || !input_dev) {
 		kfree(omap_kp);