cputime: Fix nsecs_to_cputime() return type cast

Even though nsec based cputime_t maps to u64, nsecs_to_cputime() must
return a cputime_t value. We want to enforce this kind of cast in order
to track down buggy manipulations of cputime_t such as direct access
of its values under wrong assumptions on its backend type (nsecs,
jiffies, etc...) by core code.

Cc: Ingo Molnar <[e-mail]>
Cc: Marcelo Tosatti <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Acked-by: Rik van Riel <[e-mail]>
Signed-off-by: Frederic Weisbecker <[e-mail]>

include/asm-generic/cputime_nsecs.h

···
 /*
  * Convert cputime <-> nanoseconds
  */
-#define nsecs_to_cputime(__nsecs)	((__force u64)(__nsecs))
+#define nsecs_to_cputime(__nsecs)	\
+	(__force cputime_t)(__nsecs)
 
 
 /*