ima: ima_api: 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 ima_template_entry {
	...
        struct ima_field_data template_data[0]; /* template related data */
};

instance = kzalloc(sizeof(struct ima_template_entry) + count * sizeof(struct ima_field_data), GFP_NOFS);

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_NOFS);

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

security/integrity/ima/ima_api.c

@@ -45,8 +45,8 @@ int ima_alloc_init_template(struct ima_event_data *event_data,
 	else
 		template_desc = ima_template_desc_current();
 
-	*entry = kzalloc(sizeof(**entry) + template_desc->num_fields *
-			 sizeof(struct ima_field_data), GFP_NOFS);
+	*entry = kzalloc(struct_size(*entry, template_data,
+				     template_desc->num_fields), GFP_NOFS);
 	if (!*entry)
 		return -ENOMEM;