s390/idal: use struct_size() in kmalloc()

Use the new struct_size() helper to keep code simple.
Signed-off-by: Nxiaolinkui <xiaolinkui@kylinos.cn>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/include/asm/idals.h

@@ -122,8 +122,7 @@ idal_buffer_alloc(size_t size, int page_order)
 
 	nr_ptrs = (size + IDA_BLOCK_SIZE - 1) >> IDA_SIZE_LOG;
 	nr_chunks = (4096 << page_order) >> IDA_SIZE_LOG;
-	ib = kmalloc(sizeof(struct idal_buffer) + nr_ptrs*sizeof(void *),
-		     GFP_DMA | GFP_KERNEL);
+	ib = kmalloc(struct_size(ib, data, nr_ptrs), GFP_DMA | GFP_KERNEL);
 	if (ib == NULL)
 		return ERR_PTR(-ENOMEM);
 	ib->size = size;