hpet: Use struct_size() in kzalloc()

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct foo {
    int stuff;
    struct boo entry[];
};

size = sizeof(struct foo) + count * sizeof(struct boo);
instance = kzalloc(size, GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can
now use the new struct_size() helper:

instance = kzalloc(struct_size(instance, entry, count), GFP_KERNEL);

Notice that, in this case, variable siz is not necessary, hence
it is removed.

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/hpet.c

@@ -842,7 +842,6 @@ int hpet_alloc(struct hpet_data *hdp)
 	struct hpet_dev *devp;
 	u32 i, ntimer;
 	struct hpets *hpetp;
-	size_t siz;
 	struct hpet __iomem *hpet;
 	static struct hpets *last;
 	unsigned long period;
@@ -860,10 +859,8 @@ int hpet_alloc(struct hpet_data *hdp)
 		return 0;
 	}
 
-	siz = sizeof(struct hpets) + ((hdp->hd_nirqs - 1) *
-				      sizeof(struct hpet_dev));
-
-	hpetp = kzalloc(siz, GFP_KERNEL);
+	hpetp = kzalloc(struct_size(hpetp, hp_dev, hdp->hd_nirqs - 1),
+			GFP_KERNEL);
 
 	if (!hpetp)
 		return -ENOMEM;