Some ARM systems have a short sched_clock() [ which needs to be fixed
too ], but this exposed a bug in the irq_time code as well, it doesn't
deal with wraps at all.

Fix the irq_time code to deal with u64 wraps by re-writing the code to
only use delta increments, which avoids the whole issue.
Reviewed-by: NVenkatesh Pallipadi <venki@google.com>
Reported-by: NMikael Pettersson <mikpe@it.uu.se>
Tested-by: NMikael Pettersson <mikpe@it.uu.se>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1292242433.6803.199.camel@twins>
Signed-off-by: NIngo Molnar <mingo@elte.hu>