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