dma-buf: add ref counting for module as exporter

dma-buf: add ref counting for module as exporter

Add reference counting on a kernel module that exports dma-buf and
implements its operations. This prevents the module from being unloaded
while DMABUF file is in use.

The original patch [1] was submitted by Tomasz Stanislawski, but this
is a simpler way to do it.

v3: call module_put() as late as possible, per gregkh's comment.
v2: move owner to struct dma_buf, and use DEFINE_DMA_BUF_EXPORT_INFO
    macro to simplify the change.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
[1]: https://lkml.org/lkml/2012/8/8/163

[backport from mainline commit 9abdffe286c1532a54d5aee31571d3029be4026c]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: I05cd9440f26222f877a8bfa2313a6b757419f682