dma-buf: Introduce fence_get_rcu_safe()

This variant of fence_get_rcu() takes an RCU protected pointer to a
fence and carefully returns a reference to the fence ensuring that it is
not reallocated as it does. This is required when mixing fences and
SLAB_DESTROY_BY_RCU - although it serves a more pedagogical function atm

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829070834.22296-6-chris@chris-wilson.co.uk