cpumask: modifiy oprofile initialization

Delta patch to f7df8ed1 for
tip/cpus4096.

Moved initialization to sync_start()/sync_stop(). No changes needed in
buffer_sync.h and oprof.c anymore.

Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>