perf: Simplify __perf_event_read

cpuctx is always active, task context is always active for
current

the previous condition verifies that if its a task context its
for current, hence we can assume ctx->is_active.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <20091120212509.000272254@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>