rhashtable: use struct_size() in kvzalloc()

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 = kvzalloc(size, GFP_KERNEL);

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

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

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>

lib/rhashtable.c

@@ -175,8 +175,7 @@ static struct bucket_table *bucket_table_alloc(struct rhashtable *ht,
 	int i;
 	static struct lock_class_key __key;
 
-	size = sizeof(*tbl) + nbuckets * sizeof(tbl->buckets[0]);
+	tbl = kvzalloc(struct_size(tbl, buckets, nbuckets), gfp);
-	tbl = kvzalloc(size, gfp);
 
 	size = nbuckets;