ixgbe: Use struct_size() helper

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/ixgbe/ixgbe_lib.c

@@ -836,12 +836,10 @@ static int ixgbe_alloc_q_vector(struct ixgbe_adapter *adapter,
 	struct ixgbe_ring *ring;
 	int node = NUMA_NO_NODE;
 	int cpu = -1;
-	int ring_count, size;
+	int ring_count;
 	u8 tcs = adapter->hw_tcs;
 
 	ring_count = txr_count + rxr_count + xdp_count;
-	size = sizeof(struct ixgbe_q_vector) +
-	       (sizeof(struct ixgbe_ring) * ring_count);
 
 	/* customize cpu for Flow Director mapping */
 	if ((tcs <= 1) && !(adapter->flags & IXGBE_FLAG_SRIOV_ENABLED)) {
@@ -855,9 +853,11 @@ static int ixgbe_alloc_q_vector(struct ixgbe_adapter *adapter,
 	}
 
 	/* allocate q_vector and rings */
-	q_vector = kzalloc_node(size, GFP_KERNEL, node);
+	q_vector = kzalloc_node(struct_size(q_vector, ring, ring_count),
+				GFP_KERNEL, node);
 	if (!q_vector)
-		q_vector = kzalloc(size, GFP_KERNEL);
+		q_vector = kzalloc(struct_size(q_vector, ring, ring_count),
+				   GFP_KERNEL);
 	if (!q_vector)
 		return -ENOMEM;