perf: Find task before event alloc

So that we can pass the task pointer to the event allocation, so that
we can use task associated data during event initialization.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20101014203625.340789919@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>