powerpc/spufs: use struct_size() in kmalloc()

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 = kmalloc(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 = kmalloc(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: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/cell/spufs/file.c

@@ -2338,9 +2338,8 @@ static int spufs_switch_log_open(struct inode *inode, struct file *file)
 		goto out;
 	}
 
-	ctx->switch_log = kmalloc(sizeof(struct switch_log) +
-		SWITCH_LOG_BUFSIZE * sizeof(struct switch_log_entry),
-		GFP_KERNEL);
+	ctx->switch_log = kmalloc(struct_size(ctx->switch_log, log,
+				  SWITCH_LOG_BUFSIZE), GFP_KERNEL);
 
 	if (!ctx->switch_log) {
 		rc = -ENOMEM;