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There's a small window of opportunity during which the adjust_lru()
function can be called with a GEM refcount of zero from the TTM
eviction code. This results in a kernel BUG().

Ensure that we don't attempt to modify the GEM shrinker lists unless
we have a GEM refcount.

Fixes: ebd4a8ec

 ("drm/i915/ttm: move shrinker management into adjust_lru")
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110085527.1033475-1-thomas.hellstrom@linux.intel.com