drm/i915: Fix potential bit_17 double-free

A userspace with multiple threads racing I915_GEM_SET_TILING to set the
tiling to I915_TILING_NONE could trigger a double free of the bit_17
bitmask.  (Or conversely leak memory on the transition to tiled.)  Move
allocation/free'ing of the bitmask within the section protected by the
obj lock.
Signed-off-by: Rob Clark <robdclark@chromium.org>
Fixes: 2850748e ("drm/i915: Pull i915_vma_pin under the vm->mutex")
Cc: <stable@vger.kernel.org> # v5.5+
[tursulin: Correct fixes tag and added cc stable.]
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230127200550.3531984-1-robdclark@gmail.com

drivers/gpu/drm/i915/gem/i915_gem_tiling.c

@@ -305,10 +305,6 @@ i915_gem_object_set_tiling(struct drm_i915_gem_object *obj,
 	spin_unlock(&obj->vma.lock);
 
 	obj->tiling_and_stride = tiling | stride;
-	i915_gem_object_unlock(obj);
-
-	/* Force the fence to be reacquired for GTT access */
-	i915_gem_object_release_mmap_gtt(obj);
 
 	/* Try to preallocate memory required to save swizzling on put-pages */
 	if (i915_gem_object_needs_bit17_swizzle(obj)) {
@@ -321,6 +317,11 @@ i915_gem_object_set_tiling(struct drm_i915_gem_object *obj,
 		obj->bit_17 = NULL;
 	}
 
+	i915_gem_object_unlock(obj);
+
+	/* Force the fence to be reacquired for GTT access */
+	i915_gem_object_release_mmap_gtt(obj);
+
 	return 0;
 }