dma-buf: Use rcu_assign_pointer() to set rcu protected pointers

Use rcu_assign_pointer() when setting an rcu protected pointer.
This gets rid of another sparse warning.

Cc: Dave Airlie <airlied@redhat.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: linaro-mm-sig@lists.linaro.org
Cc: linux-media@vger.kernel.org
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171102200336.23347-5-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Christian König <christian.koenig@amd.com&gt;.>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>