drm/i915: Use the correct destructor for freeing requests on error

After allocating from the slab cache, we then need to free the request
back into the slab cache upon error (and not call kfree as that leads
to eventual memory corruption).

Fixes regression from
commit efab6d8dd158fdccbe6a030f89fbf9ca0a9564e4
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Apr 7 16:20:57 2015 +0100

    drm/i915: Use a separate slab for requests

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2653,10 +2653,8 @@ int i915_gem_request_alloc(struct intel_engine_cs *ring,
 	req->i915 = dev_priv;
 
 	ret = i915_gem_get_seqno(ring->dev, &req->seqno);
-	if (ret) {
-		kfree(req);
-		return ret;
-	}
+	if (ret)
+		goto err;
 
 	req->ring = ring;
 
@@ -2664,13 +2662,15 @@ int i915_gem_request_alloc(struct intel_engine_cs *ring,
 		ret = intel_logical_ring_alloc_request_extras(req, ctx);
 	else
 		ret = intel_ring_alloc_request_extras(req);
-	if (ret) {
-		kfree(req);
-		return ret;
-	}
+	if (ret)
+		goto err;
 
 	ring->outstanding_lazy_request = req;
 	return 0;
+
+err:
+	kmem_cache_free(dev_priv->requests, req);
+	return ret;
 }
 
 struct drm_i915_gem_request *