sched_clock: fix cpu_clock()

This patch fixes 3 issues:

a) it removes the dependency on jiffies, because jiffies are incremented
   by a single CPU, and the tick is not synchronized between CPUs. Therefore
   relying on it to calculate a window to clip whacky TSC values doesn't work
   as it can drift around.

   So instead use [GTOD, GTOD+TICK_NSEC) as the window.

b) __update_sched_clock() did (roughly speaking):

   delta = sched_clock() - scd->tick_raw;
   clock += delta;

   Which gives exponential growth, instead of linear.

c) allows the sched_clock_cpu() value to warp the u64 without breaking.

the results are more reliable sched_clock() deltas:

           before       after   sched_clock

cpu_clock: 15750        51312   51488
cpu_clock: 59719        51052   50947
cpu_clock: 15879        51249   51061
cpu_clock: 1            50933   51198
cpu_clock: 1            50931   51039
cpu_clock: 1            51093   50981
cpu_clock: 1            51043   51040
cpu_clock: 1            50959   50938
cpu_clock: 1            50981   51011
cpu_clock: 1            51364   51212
cpu_clock: 1            51219   51273
cpu_clock: 1            51389   51048
cpu_clock: 1            51285   51611
cpu_clock: 1            50964   51137
cpu_clock: 1            50973   50968
cpu_clock: 1            50967   50972
cpu_clock: 1            58910   58485
cpu_clock: 1            51082   51025
cpu_clock: 1            50957   50958
cpu_clock: 1            50958   50957
cpu_clock: 1006128      51128   50971
cpu_clock: 1            51107   51155
cpu_clock: 1            51371   51081
cpu_clock: 1            51104   51365
cpu_clock: 1            51363   51309
cpu_clock: 1            51107   51160
cpu_clock: 1            51139   51100
cpu_clock: 1            51216   51136
cpu_clock: 1            51207   51215
cpu_clock: 1            51087   51263
cpu_clock: 1            51249   51177
cpu_clock: 1            51519   51412
cpu_clock: 1            51416   51255
cpu_clock: 1            51591   51594
cpu_clock: 1            50966   51374
cpu_clock: 1            50966   50966
cpu_clock: 1            51291   50948
cpu_clock: 1            50973   50867
cpu_clock: 1            50970   50970
cpu_clock: 998306       50970   50971
cpu_clock: 1            50971   50970
cpu_clock: 1            50970   50970
cpu_clock: 1            50971   50971
cpu_clock: 1            50970   50970
cpu_clock: 1            51351   50970
cpu_clock: 1            50970   51352
cpu_clock: 1            50971   50970
cpu_clock: 1            50970   50970
cpu_clock: 1            51321   50971
cpu_clock: 1            50974   51324

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>