ceph: use struct_size() helper in __ceph_pool_perm_get()

Use struct_size() to calculate the number of bytes to be allocated.
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/addr.c

@@ -2145,7 +2145,7 @@ static int __ceph_pool_perm_get(struct ceph_inode_info *ci,
 	}
 
 	pool_ns_len = pool_ns ? pool_ns->len : 0;
-	perm = kmalloc(sizeof(*perm) + pool_ns_len + 1, GFP_NOFS);
+	perm = kmalloc(struct_size(perm, pool_ns, pool_ns_len + 1), GFP_NOFS);
 	if (!perm) {
 		err = -ENOMEM;
 		goto out_unlock;