drm/xe: Fix two bo pin/unpin accounting bugs

Two independent fixes uncovered while auditing the bo pin/unpin and shrinker accounting paths:

1/2 fixes a double-subtraction of the shrinker's page/object accounting on nested xe_bo_pin_external()/xe_bo_unpin_external() calls, which can desynchronize the shrinker's counts from reality and, due to a signed-to-unsigned interpretation in xe_shrinker_count(), turn into a huge bogus shrinkable/purgeable page count that causes the shrinker to be invoked excessively under memory pressure.

2/2 fixes a stale xe->pinned.late.external list entry left behind when a bo pinned both externally and as an fb has its final unpin performed via the fb-pin path, which bypasses pinned_link maintenance and can lead to list corruption or a use-after-free once the bo is freed.

Both are tagged for stable.

v2:
- Added 2/2, fixing a stale pinned_link entry found by Sashiko AI review.
- Minor cleanup in 1/2.

v3:
- Fixed an additional shrinker accounting underflow in 1/2, affecting imported dma-bufs, found by Sashiko AI review.
- Rebased 2/2 on top of 1/2's changes.

Thomas Hellström (2):
drm/xe: Fix shrinker accounting double-subtraction on nested external
pins
drm/xe: Fix stale pinned_link entry when fb-pin performs the final
unpin

 drivers/gpu/drm/xe/display/xe_fb_pin.c |   6 +-
 drivers/gpu/drm/xe/xe_bo.c             | 139 +++++++++++++++++++------
 drivers/gpu/drm/xe/xe_bo.h             |   2 +
 3 files changed, 110 insertions(+), 37 deletions(-)

2.55.0