Commit cb9f753a3731 ("mm: fix races between swapoff and flush dcache")
updated flush_dcache_page implementations on several architectures to
use page_mapping_file() in order to avoid races between page_mapping()
and swapoff().
This update missed arch/nds32 and there is a possibility of a race
there.
Replace page_mapping() with page_mapping_file() in nds32 implementation
of flush_dcache_page().