f2fs: Convert to release_folio

While converting f2fs_release_page() to f2fs_release_folio(), cache the
sb_info so we don't need to retrieve it twice, and remove the redundant
call to set_page_private().  The use of folios should be pushed further
into f2fs from here.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>