perf, x86: Clean up reserve_ds_buffers() signature

Now that reserve_ds_buffers() never fails, change it to return
void and remove all code dealing with the error return.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Stephane Eranian <eranian@google.com>
LKML-Reference: <20101019134808.462621937@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>