buffer: use b_folio in touch_buffer()

Removes a call to compound_head() in this path.

Link: https://lkml.kernel.org/r/20221215214402.3522366-4-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>