firewire: Annotate struct fw_iso_packet 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 via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/ZgIrOuR3JI/jzqoH@neatSigned-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>

include/linux/firewire.h

@@ -463,7 +463,8 @@ struct fw_iso_packet {
 	u32 tag:2;		/* tx: Tag in packet header		*/
 	u32 sy:4;		/* tx: Sy in packet header		*/
 	u32 header_length:8;	/* Length of immediate header		*/
-	u32 header[];		/* tx: Top of 1394 isoch. data_block	*/
+				/* tx: Top of 1394 isoch. data_block    */
+	u32 header[] __counted_by(header_length);
 };
 
 #define FW_ISO_CONTEXT_TRANSMIT			0