tests: add gem_flink_race

tests: add gem_flink_race

This exercises a race in the flink name descruction of the current drm
gem core. When racing a gem close with a gem open the open can sneak
in and cause the kernel to leak the flink name and its reference.

This results in leaked gem objects that won't get reaped even at drm
file close time. On my 2 core/4 threads snb machine this leaks on the
order of 1k objects per second.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>