sched: keep utime/stime monotonic

cpustats use utime/stime as a ratio against sum_exec_runtime, as a
consequence it can happen - when the ratio changes faster than time
accumulates - that either can be appear to go backwards.

Combined backport for 2.6.23 of the following patches from mainline:
commit 73a2bcb0
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
  sched: keep utime/stime monotonic

commit 9301899b
Author: Balbir Singh <balbir@linux.vnet.ibm.com>
  sched: fix /proc/<PID>/stat stime/utime monotonicity, part 2
Signed-off-by: Frans Pop <elendil@planet.nl>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>