btrfs: Use memset_startat() to clear end of struct

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Use memset_startat() so memset() doesn't get confused about writing
beyond the destination member that is intended to be the starting point
of zeroing through the end of the struct.

Cc: Chris Mason <clm@fb.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: David Sterba <dsterba@suse.com>
Cc: linux-btrfs@vger.kernel.org
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Acked-by: David Sterba <dsterba@suse.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

fs/btrfs/root-tree.c

@@ -39,10 +39,8 @@ static void btrfs_read_root_item(struct extent_buffer *eb, int slot,
 		need_reset = 1;
 	}
 	if (need_reset) {
-		memset(&item->generation_v2, 0,
-			sizeof(*item) - offsetof(struct btrfs_root_item,
-					generation_v2));
-
+		/* Clear all members from generation_v2 onwards. */
+		memset_startat(item, 0, generation_v2);
 		generate_random_guid(item->uuid);
 	}
 }