block: genhd: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, replace the following form:

sizeof(*new_ptbl) + target * sizeof(new_ptbl->part[0])

with:

struct_size(new_ptbl, part, target)

Also, notice that variable size is unnecessary, hence it is removed.

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -1281,7 +1281,6 @@ int disk_expand_part_tbl(struct gendisk *disk, int partno)
 	struct disk_part_tbl *new_ptbl;
 	int len = old_ptbl ? old_ptbl->len : 0;
 	int i, target;
-	size_t size;
 
 	/*
 	 * check for int overflow, since we can get here from blkpg_ioctl()
@@ -1298,8 +1297,8 @@ int disk_expand_part_tbl(struct gendisk *disk, int partno)
 	if (target <= len)
 		return 0;
 
-	size = sizeof(*new_ptbl) + target * sizeof(new_ptbl->part[0]);
-	new_ptbl = kzalloc_node(size, GFP_KERNEL, disk->node_id);
+	new_ptbl = kzalloc_node(struct_size(new_ptbl, part, target), GFP_KERNEL,
+				disk->node_id);
 	if (!new_ptbl)
 		return -ENOMEM;