compiler.h: Avoid nested statement expression in data_race()

It appears that compilers have trouble with nested statement
expressions. Therefore, remove one level of statement expression nesting
from the data_race() macro. This will help avoiding potential problems
in the future as its usage increases.
Reported-by: Borislav Petkov <bp@suse.de>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lkml.kernel.org/r/20200520221712.GA21166@zn.tnic
Link: https://lkml.kernel.org/r/20200521142047.169334-10-elver@google.com

include/linux/compiler.h

@@ -264,12 +264,12 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
  */
 #define data_race(expr)							\
 ({									\
+	__unqual_scalar_typeof(({ expr; })) __v = ({			\
 		__kcsan_disable_current();				\
-	({								\
-		__unqual_scalar_typeof(({ expr; })) __v = ({ expr; });	\
+		expr;							\
+	});								\
 	__kcsan_enable_current();					\
 	__v;								\
-	});								\
 })
 
 /*