fs-verity: Use struct_size() helper in enable_verity()

Follow the best practice for allocating a variable-sized structure.
Signed-off-by: Zhang Jianhua <chris.zjh@huawei.com>
[ebiggers: adjusted commit message]
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20220519022450.2434483-1-chris.zjh@huawei.com

fs/verity/enable.c

@@ -202,7 +202,7 @@ static int enable_verity(struct file *filp,
 	const struct fsverity_operations *vops = inode->i_sb->s_vop;
 	struct merkle_tree_params params = { };
 	struct fsverity_descriptor *desc;
-	size_t desc_size = sizeof(*desc) + arg->sig_size;
+	size_t desc_size = struct_size(desc, signature, arg->sig_size);
 	struct fsverity_info *vi;
 	int err;