UPSTREAM: 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.

Change-Id: I15e947f25f71c6c712e9fe4ba03e185dc539d914
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit c910a64b)