radix_tree_for_each_slot() wants an __rcu annotated pointer for the
slot. So let's add the annotation.

Fixes the following sparse warnings:
i915_gem.c:2217:9: warning: incorrect type in assignment (different address spaces)
i915_gem.c:2217:9:    expected void **slot
i915_gem.c:2217:9:    got void [noderef] <asn:4>**
i915_gem.c:2217:9: warning: incorrect type in assignment (different address spaces)
i915_gem.c:2217:9:    expected void **slot
i915_gem.c:2217:9:    got void [noderef] <asn:4>**
i915_gem.c:2217:9: warning: incorrect type in argument 1 (different address spaces)
i915_gem.c:2217:9:    expected void [noderef] <asn:4>**slot
i915_gem.c:2217:9:    got void **slot
i915_gem.c:2217:9: warning: incorrect type in assignment (different address spaces)
i915_gem.c:2217:9:    expected void **slot
i915_gem.c:2217:9:    got void [noderef] <asn:4>**

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 96d77634 ("drm/i915: Use a radixtree for random access to the object's backing storage")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901171252.31025-1-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>