It's lockless, and userspace might chance it underneath us. That's not
really a problem, all userspace gets is a slightly dysfunctional
lease with the current code. But this might change, and gcc might
decide to reload a few too many times, and then boom. So better safe
than sorry.

v2: Remove the now unused lessor_priv argument from validate_lease()
(Keith).

v3: Actually add everything ... silly me.

Cc: Keith Packard <keithp@keithp.com>
Cc: Dave Airlie <airlied@gmail.com>
Acked-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181105101211.22737-1-daniel.vetter@ffwll.ch