perf_counter: rework the task clock software counter

Rework the task clock software counter to use the context time instead
of the task runtime clock, this removes the last such user.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
LKML-Reference: <20090406094518.445450972@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>