xen/gntdev: Do not destroy context while dma-bufs are in use

If there are exported DMA buffers which are still in use and
grant device is closed by either normal user-space close or by
a signal this leads to the grant device context to be destroyed,
thus making it not possible to correctly destroy those exported
buffers when they are returned back to gntdev and makes the module
crash:

[  339.617540] [<ffff00000854c0d8>] dmabuf_exp_ops_release+0x40/0xa8
[  339.617560] [<ffff00000867a6e8>] dma_buf_release+0x60/0x190
[  339.617577] [<ffff0000082211f0>] __fput+0x88/0x1d0
[  339.617589] [<ffff000008221394>] ____fput+0xc/0x18
[  339.617607] [<ffff0000080ed4e4>] task_work_run+0x9c/0xc0
[  339.617622] [<ffff000008089714>] do_notify_resume+0xfc/0x108

Fix this by referencing gntdev on each DMA buffer export and
unreferencing on buffer release.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Boris <Ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>