firewire: Annotate struct fw_node with __counted_by

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 checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct fw_node.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Takashi Sakamoto <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Gustavo A. R. Silva <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Takashi Sakamoto <[email protected]>

Author: kees

drivers/firewire/core.h

@@ -191,7 +191,7 @@ struct fw_node {
 	/* Upper layer specific data. */
 	void *data;
 
-	struct fw_node *ports[];
+	struct fw_node *ports[] __counted_by(port_count);
 };
 
 static inline struct fw_node *fw_node_get(struct fw_node *node)