buffer: use b_folio in end_buffer_async_read()

Removes a call to compound_head() in SetPageError(), saving 76 bytes of
text.

Link: https://lkml.kernel.org/r/20221215214402.3522366-5-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>