perf_counter: fix NMI race in task clock

We should not be updating ctx->time from NMI context, work around that.

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: <20090408130408.681326666@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>