f2fs: Hoist the page_folio() call to the start of f2fs_merge_page_bio()

Remove one call to compound_head() and a reference to page->mapping
by calling page_folio() early on.

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