mlxsw: spectrum_kvdl: Use struct_size() helper in kzalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/net/ethernet/mellanox/mlxsw/spectrum1_kvdl.c:229:24: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlxsw/spectrum1_kvdl.c

@@ -213,7 +213,6 @@ mlxsw_sp1_kvdl_part_init(struct mlxsw_sp *mlxsw_sp,
 	struct mlxsw_sp1_kvdl_part *part;
 	bool need_update = true;
 	unsigned int nr_entries;
-	size_t usage_size;
 	u64 resource_size;
 	int err;
 
@@ -225,8 +224,8 @@ mlxsw_sp1_kvdl_part_init(struct mlxsw_sp *mlxsw_sp,
 	}
 
 	nr_entries = div_u64(resource_size, info->alloc_size);
-	usage_size = BITS_TO_LONGS(nr_entries) * sizeof(unsigned long);
-	part = kzalloc(sizeof(*part) + usage_size, GFP_KERNEL);
+	part = kzalloc(struct_size(part, usage, BITS_TO_LONGS(nr_entries)),
+		       GFP_KERNEL);
 	if (!part)
 		return ERR_PTR(-ENOMEM);