f2fs: Remove calls to folio_file_mapping()

All folios that f2fs sees belong to f2fs and not to the swapcache
so it can dereference folio->mapping directly like all other
filesystems do.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>