fm10k: 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 size 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: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/fm10k/fm10k_main.c

@@ -1603,14 +1603,12 @@ static int fm10k_alloc_q_vector(struct fm10k_intfc *interface,
 {
 	struct fm10k_q_vector *q_vector;
 	struct fm10k_ring *ring;
-	int ring_count, size;
+	int ring_count;
 
 	ring_count = txr_count + rxr_count;
-	size = sizeof(struct fm10k_q_vector) +
-	       (sizeof(struct fm10k_ring) * ring_count);
 
 	/* allocate q_vector and rings */
-	q_vector = kzalloc(size, GFP_KERNEL);
+	q_vector = kzalloc(struct_size(q_vector, ring, ring_count), GFP_KERNEL);
 	if (!q_vector)
 		return -ENOMEM;