f2fs: Convert fsync_node_entry->page to folio

Convert all callers to set/get a folio instead of a page.  Removes
five 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>