f2fs: Convert truncate_partial_data_page() to use a folio

Retrieve a folio from the page cache and use it throughout.
Saves five hidden calls to compound_head().

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