drm/ttm: Fix a NULL pointer dereference

The LRU mechanism may look up a resource in the process of being removed
from an object. The locking rules here are a bit unclear but it looks
currently like res->bo assignment is protected by the LRU lock, whereas
bo->resource is protected by the object lock, while *clearing* of
bo->resource is also protected by the LRU lock. This means that if
we check that bo->resource points to the LRU resource under the LRU
lock we should be safe.
So perform that check before deciding to swap out a bo. That avoids
dereferencing a NULL bo->resource in ttm_bo_swapout().

Fixes: 6a9b02899402 ("drm/ttm: move the LRU into resource handling v4")
Cc: Christian König <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Christian Koenig <[email protected]>
Cc: Huang Rui <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Felix Kuehling <[email protected]>
Cc: Philip Yang <[email protected]>
Cc: Qiang Yu <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Nirmoy Das <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: "Thomas Hellström" <[email protected]>
Cc: Anshuman Gupta <[email protected]>
Cc: Arunpravin Paneer Selvam <[email protected]>
Cc: [email protected]
Cc: <[email protected]> # v5.19+
Signed-off-by: Thomas Hellström <[email protected]>
Reviewed-by: Christian König <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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