drm/i915: Also drop vm.ref along error paths for vma construction

Not only do we need to release the vm.ref we acquired for the vma on the
duplicate insert branch, but also for the normal error paths, so roll
them all into one.

Reported-by: Andi Shyti <andi.shyti@intel.com>
Suggested-by: Andi Shyti <andi.shyti@intel.com>
Fixes: 2850748 ("drm/i915: Pull i915_vma_pin under the vm->mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: <stable@vger.kernel.org> # v5.5+
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702211015.29604-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_vma.c

@@ -104,6 +104,7 @@ vma_create(struct drm_i915_gem_object *obj,
 	   struct i915_address_space *vm,
 	   const struct i915_ggtt_view *view)
 {
+	struct i915_vma *pos = ERR_PTR(-E2BIG);
 	struct i915_vma *vma;
 	struct rb_node *rb, **p;
 
@@ -184,7 +185,6 @@ vma_create(struct drm_i915_gem_object *obj,
 	rb = NULL;
 	p = &obj->vma.tree.rb_node;
 	while (*p) {
-		struct i915_vma *pos;
 		long cmp;
 
 		rb = *p;
@@ -196,17 +196,12 @@ vma_create(struct drm_i915_gem_object *obj,
 		 * and dispose of ours.
 		 */
 		cmp = i915_vma_compare(pos, vm, view);
-		if (cmp == 0) {
-			spin_unlock(&obj->vma.lock);
-			i915_vm_put(vm);
-			i915_vma_free(vma);
-			return pos;
-		}
-
 		if (cmp < 0)
 			p = &rb->rb_right;
-		else
+		else if (cmp > 0)
 			p = &rb->rb_left;
+		else
+			goto err_unlock;
 	}
 	rb_link_node(&vma->obj_node, rb, p);
 	rb_insert_color(&vma->obj_node, &obj->vma.tree);
@@ -229,8 +224,9 @@ vma_create(struct drm_i915_gem_object *obj,
 err_unlock:
 	spin_unlock(&obj->vma.lock);
 err_vma:
+	i915_vm_put(vm);
 	i915_vma_free(vma);
-	return ERR_PTR(-E2BIG);
+	return pos;
 }
 
 static struct i915_vma *