drm/nouveau: fix stale locked mutex in nouveau_gem_ioctl_pushbuf

In the Linux kernel, the following vulnerability has been resolved: 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.