message/i2o: add sanity check

We use "reply_size" to determine the size of the "reply" buffer. These days the "reply" buffer is just some context information, an error code, and a bunch of zeroes. We stopped using it for useful information four years ago in dcceafe25a5f47 "[PATCH] I2O: Bugfixes"

This patch ensures that "reply" is large enough to hold the context
information. In the current code if "reply_size" is zero that would
cause memory corruption when we save the error code:

	reply[4] = ((u32) rcode) << 24;

index c4b117f..8b1d55e 100644
@@ -817,6 +817,11 @@ static int i2o_cfg_passthru(unsigned long arg)
 	reply_size >>= 16;
 	reply_size <<= 2;
 
+	if (reply_size < 5 * sizeof(u32)) {
+		rcode = -EINVAL;
+		goto out;
+	}
+
 	reply = kzalloc(reply_size, GFP_KERNEL);
 	if (!reply) {
 		printk(KERN_WARNING "%s: Could not allocate reply buffer\n",