drm/nouveau: Add a dedicated mutex for the clients list

drm/nouveau: Add a dedicated mutex for the clients list

commit abae9164a421bc4a41a3769f01ebcd1f9d955e0e upstream.

Rather than protecting the nouveau_drm clients list with the lock within
the "client" nouveau_cli, add a dedicated lock to serialize access to
the list. This is both clearer and necessary to avoid lockdep being
upset with us when we need to iterate through all the clients in the
list and potentially lock their mutex, which is the same class as the
lock protecting the entire list.

Cc: stable@vger.kernel.org # 5.4+
Signed-off-by: Jeremy Cline <jcline@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Tested-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201125202648.5220-3-jcline@redhat.com
Link: https://gitlab.freedesktop.org/drm/nouveau/-/merge_requests/14
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>