i2c: mux: harden i2c_mux_alloc() against integer overflows

A couple years back we went through the kernel an automatically
converted size calculations to use struct_size() instead.  The
struct_size() calculation is protected against integer overflows.

However it does not make sense to use the result from struct_size()
for additional math operations as that would negate any safeness.

Fixes: 1f3b69b ("i2c: mux: Use struct_size() in devm_kzalloc()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

Author: Dan Carpenter

drivers/i2c/i2c-mux.c

@@ -243,9 +243,10 @@ struct i2c_mux_core *i2c_mux_alloc(struct i2c_adapter *parent,
 				   int (*deselect)(struct i2c_mux_core *, u32))
 {
 	struct i2c_mux_core *muxc;
+	size_t mux_size;
 
-	muxc = devm_kzalloc(dev, struct_size(muxc, adapter, max_adapters)
-			    + sizeof_priv, GFP_KERNEL);
+	mux_size = struct_size(muxc, adapter, max_adapters);
+	muxc = devm_kzalloc(dev, size_add(mux_size, sizeof_priv), GFP_KERNEL);
 	if (!muxc)
 		return NULL;
 	if (sizeof_priv)