ion: Switch ion to use dma-buf

Ion now uses dma-buf file descriptors to share
buffers with userspace.  Ion becomes a dma-buf
exporter and any driver that can import dma-bufs
can now import ion file descriptors.

Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>