btrfs: add __counted_by for struct btrfs_delayed_item and use struct_size()

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

While there, use struct_size() helper, instead of the open-coded
version, to calculate the size for the allocation of the whole
flexible structure, including of course, the flexible-array member.

This code was found with the help of Coccinelle, and audited and
fixed manually.
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/delayed-inode.c

@@ -313,7 +313,7 @@ static struct btrfs_delayed_item *btrfs_alloc_delayed_item(u16 data_len,
 {
 	struct btrfs_delayed_item *item;
 
-	item = kmalloc(sizeof(*item) + data_len, GFP_NOFS);
+	item = kmalloc(struct_size(item, data, data_len), GFP_NOFS);
 	if (item) {
 		item->data_len = data_len;
 		item->type = type;

fs/btrfs/delayed-inode.h

@@ -95,7 +95,7 @@ struct btrfs_delayed_item {
 	bool logged;
 	/* The maximum leaf size is 64K, so u16 is more than enough. */
 	u16 data_len;
-	char data[];
+	char data[] __counted_by(data_len);
 };
 
 static inline void btrfs_init_delayed_root(