sched: Sanitize irq accounting madness

Russell reported, that irqtime_account_idle_ticks() takes ages due to:

       for (i = 0; i < ticks; i++)
               irqtime_account_process_tick(current, 0, rq);

It's sad, that this code was written way _AFTER_ the NOHZ idle
functionality was available. I charge myself guitly for not paying
attention when that crap got merged with commit abb74cef ("sched:
Export ns irqtimes through /proc/stat")

So instead of looping nr_ticks times just apply the whole thing at
once.

As a side note: The whole cputime_t vs. u64 business in that context
wants to be cleaned up as well. There is no point in having all these
back and forth conversions. Lets standardise on u64 nsec for all
kernel internal accounting and be done with it. Everything else does
not make sense at all for fine grained accounting. Frederic, can you
please take care of that?

Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Venkatesh Pallipadi <venki@google.com>
Cc: Shaun Ruffell <sruffell@digium.com>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/alpine.DEB.2.02.1405022307000.6261@ionos.tec.linutronix.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>