staging: erofs: introduce erofs_page_is_managed()

1) In order to clean up unnecessary
   page->mapping == MNGD_MAPPING(sbi) wrapped by #ifdefs;

2) Needed by "staging: erofs: support IO read error injection".

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>