block: Make BlockDriver method bdrv_set_locked() return void The only caller is bdrv_set_locked(), and it ignores the value. Callees always return 0, except for FreeBSD's cdrom_set_locked(), which returns -ENOTSUP when the device is in a terminally wedged state. Signed-off-by: Markus Armbruster <armbru@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>