dma-buf: add get_dma_buf()

Works in a similar way to get_file(), and is needed in cases such as
when the exporter needs to also keep a reference to the dmabuf (that
is later released with a dma_buf_put()), and possibly other similar
cases.

Change-Id: Ib88d185acab6503ed4bda7ba2e2c73aaee8821c1
Signed-off-by: Rob Clark <rob@ti.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>