tracing/synthetic: Fix order of struct trace_dynamic_info

To make handling BIG and LITTLE endian better the offset/len of dynamic
fields of the synthetic events was changed into a structure of:

 struct trace_dynamic_info {
 #ifdef CONFIG_CPU_BIG_ENDIAN
	u16	offset;
	u16	len;
 #else
	u16	len;
	u16	offset;
 #endif
 };

to replace the manual changes of:

 data_offset = offset & 0xffff;
 data_offest = len << 16;

But if you look closely, the above is:

  <len> << 16 | offset

Which in little endian would be in memory:

 offset_lo offset_hi len_lo len_hi

and in big endian:

 len_hi len_lo offset_hi offset_lo

Which if broken into a structure would be:

 struct trace_dynamic_info {
 #ifdef CONFIG_CPU_BIG_ENDIAN
	u16	len;
	u16	offset;
 #else
	u16	offset;
	u16	len;
 #endif
 };

Which is the opposite of what was defined.

Fix this and just to be safe also add "__packed".

Link: https://lore.kernel.org/all/20230908154417.5172e343@gandalf.local.home/
Link: https://lore.kernel.org/linux-trace-kernel/20230908163929.2c25f3dc@gandalf.local.home

Cc: stable@vger.kernel.org
Cc: Mark Rutland <mark.rutland@arm.com>
Tested-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Fixes: ddeea494a1 ("tracing/synthetic: Use union instead of casts")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

include/linux/trace_events.h

@@ -62,13 +62,13 @@ void trace_event_printf(struct trace_iterator *iter, const char *fmt, ...);
 /* Used to find the offset and length of dynamic fields in trace events */
 struct trace_dynamic_info {
 #ifdef CONFIG_CPU_BIG_ENDIAN
-	u16	offset;
 	u16	len;
+	u16	offset;
 #else
-	u16	len;
 	u16	offset;
+	u16	len;
 #endif
-};
+} __packed;
 
 /*
  * The trace entry - the most basic unit of tracing. This is what