[S390] cputime: add sparse checking and cleanup

Make cputime_t and cputime64_t nocast to enable sparse checking to
detect incorrect use of cputime. Drop the cputime macros for simple
scalar operations. The conversion macros are still needed.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

include/linux/sched.h

@@ -483,8 +483,8 @@ struct task_cputime {
 
 #define INIT_CPUTIME	\
 	(struct task_cputime) {					\
-		.utime = cputime_zero,				\
-		.stime = cputime_zero,				\
+		.utime = 0,					\
+		.stime = 0,					\
 		.sum_exec_runtime = 0,				\
 	}