drm/xe: Fix shrinker accounting double-subtraction on nested external pins

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

drm/xe: Fix shrinker accounting double-subtraction on nested external pins

xe_bo_pin_external() and xe_bo_unpin_external() support nested pins,
since the underlying ttm_bo_pin()/ttm_bo_unpin() maintain a pin_count
refcount rather than a boolean. However, the shrinker accounting calls
xe_ttm_tt_account_subtract() and xe_ttm_tt_account_add() are invoked
unconditionally on every pin_external()/unpin_external() call, instead
of only on the transition into and out of the pinned state.

An external BO (dma-buf) can be pinned more than once while already
pinned, for example when it has multiple attachments/importers, each
independently calling xe_bo_pin_external(). Each such nested pin
subtracts the BO's pages from the shrinker's accounting again, even
though the BO's pages were already removed from consideration by the
first pin. Symmetrically, each nested unpin adds them back again
before the BO is actually unpinned. This desynchronizes the
shrinker's page and object counts from reality, and since they are
declared as signed long but interpreted as unsigned long in
xe_shrinker_count(), the underflow can turn into an enormous
shrinkable/purgeable page count, causing the shrinker to be invoked
excessively under memory pressure.

Guard the accounting calls with the same pin-count transition checks
already used to guard the pinned_link list maintenance, so accounting
is only updated on the outermost pin and the final unpin.

While centralizing this logic into xe_bo_account_pin()/
xe_bo_account_unpin(), also close a second, related accounting gap
affecting imported dma-bufs. For an imported bo's ttm_tt, xe's
ttm_tt_populate hook (xe_ttm_tt_populate()) early-returns without
calling xe_ttm_tt_account_add(), since TTM_TT_FLAG_EXTERNAL is set
without TTM_TT_FLAG_EXTERNAL_MAPPABLE. However, TTM core's
ttm_tt_populate() wrapper unconditionally marks the tt as populated
on driver-hook success, so ttm_tt_is_populated() still reports true
for it. xe_bo_account_pin()/xe_bo_account_unpin() trusted
ttm_tt_is_populated() as a proxy for "these pages are accounted for
by the shrinker", and would subtract/add pages for such a tt that
were never added in the first place, again underflowing the
shrinker's counts. This is reachable today via xe_bo_pin_external(),
and would have become newly reachable via the fb-pin path once this
patch routes __xe_pin_fb_vma() through xe_bo_pin_account() below, so
fix it here rather than in a later patch, to avoid a commit in this
series that introduces the regression before fixing it. Skip the
accounting for imported bos, using the same EXTERNAL &&
!EXTERNAL_MAPPABLE test xe_ttm_tt_populate()/xe_ttm_tt_unpopulate()
use to decide whether to act, via a new xe_ttm_bo_is_imported()
helper (hoisted up from its only prior use site).

v2:
- Fix the critical issue caught by Sashiko AI review: the pin-count
transition check introduced here was still bypassed by the fb-pin
path (__xe_pin_fb_vma()/__xe_unpin_fb_vma()), which could leave a
stale pinned_link entry and a permanent shrinker accounting leak
when the fb-unpin performed the true last unpin. Addressed in a
separate follow-up patch in this series rather than folded in here,
since it is a distinct bug with its own Fixes: tag.
- Drop the unneeded local "last_unpin" variable in
xe_bo_unpin_external(), restoring the original inline
bo->ttm.pin_count == 1 check; it was a no-op rename not used by any
of the new accounting helpers, and was going to be deleted again by
the follow-up patch anyway.

v3:
- Exclude imported dma-bufs from the pin/unpin shrinker accounting,
fixing another underflow caught by Sashiko AI review.
- Add a short comment on the backup-object pin site noting it's
already populated/accounted at pin time, addressing (as a
false-positive clarification, no code fix needed) a separate
Medium-severity accounting-leak concern also caught by Sashiko AI

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