CGroups _s64 files: use read_s64/write_s64 in CFS cgroup for rt_runtime file

This removes some filesystem boilerplate from the CFS cgroup subsystem.

Signed-off-by: Paul Menage <menage@google.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>