f2fs: Remove use of page from f2fs_write_single_data_page()

Both remaining uses of page now have a folio equivalent.

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