commit 1c949842 upstream.

While developing MST support I noticed I often got the wrong data
back from a transaction, in a racy fashion. I noticed the scratch
space wasn't locked against concurrent users.

Based on a patch by Alex, but I've made it a bit more obvious when
things are locked.
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>