compiler.h: simplify data_race() macro

-Wdeclaration-after-statement used since forever required statement
expressions to inject __kcsan_disable_current(), __kcsan_enable_current()
to mark data race. Now that it is gone, make macro expansion simpler.

__unqual_scalar_typeof() is wordy macro by itself.
"expr" is expanded twice.

Link: https://lkml.kernel.org/r/fb62163f-ba21-4661-be5b-bb5124abc87d@p183
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Alexey Dobriyan

include/linux/compiler.h

@@ -200,10 +200,8 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
  */
 #define data_race(expr)							\
 ({									\
-	__unqual_scalar_typeof(({ expr; })) __v = ({			\
-		__kcsan_disable_current();				\
-		expr;							\
-	});								\
+	__kcsan_disable_current();					\
+	__auto_type __v = (expr);					\
 	__kcsan_enable_current();					\
 	__v;								\
 })