buffer: use b_folio in end_buffer_async_write()

Save 76 bytes from avoiding the call to compound_head() in SetPageError().
Also avoid the call to compound_head() in end_page_writeback().

Link: https://lkml.kernel.org/r/20221215214402.3522366-6-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>