drm/ttm: Fix a NULL pointer dereference

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

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().