drm/i915/gem: Correct the locking and pin pattern for dma-buf (v8)

If our exported dma-bufs are imported by another instance of our driver,
that instance will typically have the imported dma-bufs locked during
dma_buf_map_attachment(). But the exporter also locks the same reservation
object in the map_dma_buf() callback, which leads to recursive locking.

So taking the lock inside _pin_pages_unlocked() is incorrect.

Additionally, the current pinning code path is contrary to the defined
way that pinning should occur.

Remove the explicit pin/unpin from the map/umap functions and move them
to the attach/detach allowing correct locking to occur, and to match
the static dma-buf drm_prime pattern.

Add a live selftest to exercise both dynamic and non-dynamic
exports.

v2:
- Extend the selftest with a fake dynamic importer.
- Provide real pin and unpin callbacks to not abuse the interface.
v3: (ruhl)
- Remove the dynamic export support and move the pinning into the
  attach/detach path.
v4: (ruhl)
- Put pages does not need to assert on the dma-resv
v5: (jason)
- Lock around dma_buf_unmap_attachment() when emulating a dynamic
  importer in the subtests.
- Use pin_pages_unlocked
v6: (jason)
- Use dma_buf_attach instead of dma_buf_attach_dynamic in the selftests
v7: (mauld)
- Use __i915_gem_object_get_pages (2 __underscores) instead of the
  4 ____underscore version in the selftests
v8: (mauld)
- Drop the kernel doc from the static i915_gem_dmabuf_attach function
- Add missing "err = PTR_ERR()" to a bunch of selftest error cases

Reported-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723172142.3273510-8-jason@jlekstrand.net