drm/xe: Hold a dma-buf reference for imported BOs

In the Linux kernel, the following vulnerability has been resolved:

drm/xe: Hold a dma-buf reference for imported BOs

An imported dma-buf BO is created as a ttm_bo_type_sg BO whose
reservation object is the exporter's dma_buf->resv. The importer,
however, only takes a dma-buf reference after a successful
dma_buf_dynamic_attach(). Until then nothing keeps the exporter alive,
so if the exporter is freed while the BO still references its resv, a
later access to that resv is a use-after-free:

Oops: general protection fault, probably for non-canonical address
0x6b6b6b6b6b6b6b9c
Workqueue: ttm ttm_bo_delayed_delete [ttm]
RIP: 0010:mutex_can_spin_on_owner+0x3f/0xc0

This can be reached on two paths:

- dma_buf_dynamic_attach() fails, or
- ttm_bo_init_reserved() fails during BO creation.

In both cases the BO already has bo->base.resv pointing at the exporter
resv, and sg BOs are always torn down via ttm_bo_delayed_delete(), which
locks bo->base.resv asynchronously - potentially after the exporter has
been freed.

Take the dma-buf reference in xe_bo_init_locked(), before
ttm_bo_init_reserved(), so it also covers a creation failure there, and
release it in xe_ttm_bo_destroy(). The reference is held for the whole
BO lifetime, keeping the shared resv alive on every path.

v2:
- Reworked the fix to avoid creating the imported sg BO before
dma_buf_dynamic_attach() succeeds.
- Attach with importer_priv == NULL and make invalidate_mappings ignore
incomplete imports.

v3:
- Dropped the xe-side reordering approach since importer_priv must be
valid when dma_buf_dynamic_attach() publishes the attachment.
- Per Christian's suggestion on the v1 thread, keyed the check on
import_attach rather than removing the sg guard entirely.
- Fixes both xe and amdgpu in a single TTM patch.

v4:
- Moved import_attach check to after dma_resv_copy_fences() so fences
are copied before returning for successful imports (Thomas).
- Removed exporter-alive claim from commit message (Thomas).

v5:
- Add drm/xe patch to keep imported sg BOs off the LRU before attach
succeeds; the TTM fix alone is not sufficient for xe if the BO is
already LRU-visible. (Thomas)
v4 patch:
https://patchwork.freedesktop.org/patch/736663/?series=169129&rev=2
- Patch 1 (drm/ttm) carries Christian's Reviewed-by from v4.

v6:
- Reworked the fix based on Thomas' suggestion. Instead of the TTM resv
individualization (v1-v5) plus the xe off-LRU/placement handling (v5),
just hold a dma-buf reference for the imported BO lifetime so the
shared resv can never be freed while the BO still references it.
Single xe patch, no TTM change. (Thomas)
- Take the reference in xe_bo_init_locked() before ttm_bo_init_reserved()
so a TTM creation failure is covered too (Thomas).
- Dropped the v5 series (drm/ttm + drm/xe off-LRU); the off-LRU approach
also regressed in CI BAT via ttm_bo_pipeline_gutting() creating a ghost
BO that outlived the exporter.
Link to v5: https://patchwork.freedesktop.org/series/169984/

v7:
- Move changelog above --- so it stays in the commit message.
- Reorder changelog entries oldest-to-newest. (Thomas)

(cherry picked from commit 3516f3fae6be35642f8f06f8a218da6425c0306a)