posix-cpu-timers: don't account cpu timer after stopped thread runtime accounting

posix-cpu-timers: don't account cpu timer after stopped thread runtime accounting

When tsk->signal->cputimer->running is 1, signal->cputimer (i.e. per process
timer account) and tsk->sum_sched_runtime (i.e. per thread timer account)
increase at the same pace because update_curr() increases both accounting.

However, there is one exception. When thread exiting, __exit_signal() turns
over task's sum_shced_runtime to sig->sum_sched_runtime, but it doesn't stop
signal->cputimer accounting.

This inconsistency makes POSIX timer wake up too early. This patch fixes it.

Original-patch-by: Olivier Langlois <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Acked-by: Peter Zijlstra <[email protected]>
Signed-off-by: Olivier Langlois <[email protected]>
Signed-off-by: KOSAKI Motohiro <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>