staging: erofs: clean up shrinker stuffs

- rename erofs_register_super / erofs_unregister_super
  to erofs_shrinker_register / erofs_shrinker_unregister;
- fold the only erofs_shrink_workstation external call
  to erofs_shrinker_unregister;
- localize erofs_shrink_workstation;
- localize erofs_shrinker_info by introducing
  erofs_init_shrinker and erofs_exit_shrinker.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-10-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>