drm/vgem: Pin our pages for dmabuf exports

When the caller maps their dmabuf and we return an sg_table, the caller
doesn't expect the pages beneath that sg_table to vanish on a whim (i.e.
under mempressure). The contract is that the pages are pinned for the
duration of the mapping (from dma_buf_map_attachment() to
dma_buf_unmap_attachment). To comply, we need to introduce our own
vgem_object.pages_pin_count and elevate it across the mapping. However,
the drm_prime interface we use calls drv->prime_pin on dma_buf_attach
and drv->prime_unpin on dma_buf_detach, which while that does cover the
mapping is much broader than is desired -- but it will do for now.

v2: also hold the pin across prime_vmap/vunmap

Reported-by: Tomi Sarvela <[email protected]>
Testcase: igt/gem_concurrent_blit/*swap*vgem*
Fixes: 5ba6c9ff961a ("drm/vgem: Fix mmaping")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Tomi Sarvela <[email protected]>
Cc: Laura Abbott <[email protected]>
Cc: Sean Paul <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: <[email protected]> # needs a backport
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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