ovl: Annotate struct ovl_entry with __counted_by()

Add the __counted_by() compiler attribute to the flexible array member
'__lowerstack' to improve access bounds-checking via CONFIG_UBSAN_BOUNDS
and CONFIG_FORTIFY_SOURCE.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: toblux

fs/overlayfs/ovl_entry.h

@@ -51,7 +51,7 @@ struct ovl_path {
 
 struct ovl_entry {
 	unsigned int __numlower;
-	struct ovl_path __lowerstack[];
+	struct ovl_path __lowerstack[] __counted_by(__numlower);
 };
 
 /* private information held for overlayfs's superblock */