x86, bts: wait until traced task has been scheduled out

In order to stop branch tracing for a running task, we need to first
clear the branch tracing control bits before we may free the tracing
buffer.

If the traced task is running, the cpu might still trace that task
after the branch trace control bits have cleared.

Wait until the traced task has been scheduled out before proceeding.

A similar problem affects the task debug store context. We first remove
the context, then we need to wait until the task has been scheduled
out before we can free the context memory.

Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Cc: roland@redhat.com
Cc: eranian@googlemail.com
Cc: juan.villacis@intel.com
Cc: ak@linux.jf.intel.com
LKML-Reference: <20090403144551.919636000@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>