memstick: jmb38x_ms: 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/20210911131933.2089-1-len.baker@gmx.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/memstick/host/jmb38x_ms.c

@@ -927,8 +927,7 @@ static int jmb38x_ms_probe(struct pci_dev *pdev,
 		goto err_out_int;
 	}
 
-	jm = kzalloc(sizeof(struct jmb38x_ms)
-		     + cnt * sizeof(struct memstick_host *), GFP_KERNEL);
+	jm = kzalloc(struct_size(jm, hosts, cnt), GFP_KERNEL);
 	if (!jm) {
 		rc = -ENOMEM;
 		goto err_out_int;