s390/hypfs: 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;
    void *entry[];
};

instance = kzalloc(sizeof(struct foo) + sizeof(void *) * count, 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);

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/hypfs/hypfs_diag0c.c

@@ -54,8 +54,7 @@ static void *diag0c_store(unsigned int *count)
 	if (!cpu_vec)
 		goto fail_put_online_cpus;
 	/* Note: Diag 0c needs 8 byte alignment and real storage */
-	diag0c_data = kzalloc(sizeof(struct hypfs_diag0c_hdr) +
-			      cpu_count * sizeof(struct hypfs_diag0c_entry),
+	diag0c_data = kzalloc(struct_size(diag0c_data, entry, cpu_count),
 			      GFP_KERNEL | GFP_DMA);
 	if (!diag0c_data)
 		goto fail_kfree_cpu_vec;