drm/nouveau: fix stale locked mutex in nouveau_gem_ioctl_pushbuf

If VM_BIND is enabled on the client the legacy submission ioctl can't be
used, however if a client tries to do so regardless it will return an
error. In this case the clients mutex remained unlocked leading to a
deadlock inside nouveau_drm_postclose or any other nouveau ioctl call.

Fixes: b88baab8 ("drm/nouveau: implement new VM_BIND uAPI")
Cc: Danilo Krummrich <dakr@redhat.com>
Cc: <stable@vger.kernel.org> # v6.6+
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240305133853.2214268-1-kherbst@redhat.com

drivers/gpu/drm/nouveau/nouveau_gem.c

@@ -764,7 +764,7 @@ nouveau_gem_ioctl_pushbuf(struct drm_device *dev, void *data,
 		return -ENOMEM;
 
 	if (unlikely(nouveau_cli_uvmm(cli)))
-		return -ENOSYS;
+		return nouveau_abi16_put(abi16, -ENOSYS);
 
 	list_for_each_entry(temp, &abi16->channels, head) {
 		if (temp->chan->chid == req->channel) {