tracing: Use alignof__(struct {type b;}) instead of offsetof()

Simplify:

  #define ALIGN_STRUCTFIELD(type) ((int)(offsetof(struct {char a; type b;}, b)))

with

  #define  ALIGN_STRUCTFIELD(type) __alignof__(struct {type b;})

Which works just the same.

Link: https://lore.kernel.org/all/a7d202457150472588df0bd3b7334b3f@AcuMS.aculab.com/
Link: https://lkml.kernel.org/r/20220802154412.513c50e3@gandalf.local.home

Suggested-by: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

include/trace/stages/stage4_event_fields.h

@@ -2,7 +2,7 @@
 
 /* Stage 4 definitions for creating trace events */
 
-#define ALIGN_STRUCTFIELD(type) ((int)(offsetof(struct {char a; type b;}, b)))
+#define ALIGN_STRUCTFIELD(type) ((int)(__alignof__(struct {type b;})))
 
 #undef __field_ext
 #define __field_ext(_type, _item, _filter_type) {			\