dma-fence/reservation: Markup rcu protected access for DEBUG_MUTEXES

Mark the access to reservation_object.fence as being protected to
silence sparse.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190612132830.31221-1-chris@chris-wilson.co.uk