bcachefs: Annotate struct bch_xattr with __counted_by()

Add the __counted_by compiler attribute to the flexible array member
x_name to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/xattr_format.h

@@ -13,7 +13,7 @@ struct bch_xattr {
 	__u8			x_type;
 	__u8			x_name_len;
 	__le16			x_val_len;
-	__u8			x_name[];
+	__u8			x_name[] __counted_by(x_name_len);
 } __packed __aligned(8);
 
 #endif /* _BCACHEFS_XATTR_FORMAT_H */