mlxsw: spectrum_router: 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 alloc_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>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c

@@ -3814,13 +3814,11 @@ mlxsw_sp_nexthop4_group_create(struct mlxsw_sp *mlxsw_sp, struct fib_info *fi)
 	struct mlxsw_sp_nexthop_group *nh_grp;
 	struct mlxsw_sp_nexthop *nh;
 	struct fib_nh *fib_nh;
-	size_t alloc_size;
 	int i;
 	int err;
 
-	alloc_size = sizeof(*nh_grp) +
-		     fi->fib_nhs * sizeof(struct mlxsw_sp_nexthop);
-	nh_grp = kzalloc(alloc_size, GFP_KERNEL);
+	nh_grp = kzalloc(struct_size(nh_grp, nexthops, fi->fib_nhs),
+			 GFP_KERNEL);
 	if (!nh_grp)
 		return ERR_PTR(-ENOMEM);
 	nh_grp->priv = fi;
@@ -5066,13 +5064,11 @@ mlxsw_sp_nexthop6_group_create(struct mlxsw_sp *mlxsw_sp,
 	struct mlxsw_sp_nexthop_group *nh_grp;
 	struct mlxsw_sp_rt6 *mlxsw_sp_rt6;
 	struct mlxsw_sp_nexthop *nh;
-	size_t alloc_size;
 	int i = 0;
 	int err;
 
-	alloc_size = sizeof(*nh_grp) +
-		     fib6_entry->nrt6 * sizeof(struct mlxsw_sp_nexthop);
-	nh_grp = kzalloc(alloc_size, GFP_KERNEL);
+	nh_grp = kzalloc(struct_size(nh_grp, nexthops, fib6_entry->nrt6),
+			 GFP_KERNEL);
 	if (!nh_grp)
 		return ERR_PTR(-ENOMEM);
 	INIT_LIST_HEAD(&nh_grp->fib_list);