splice: Convert page_cache_pipe_buf_confirm() to use a folio

Convert buf->page to a folio once instead of five times.  There's only
one uptodate bit per folio, not per page, so we lose nothing here.

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Message-Id: <20230821141541.2535953-1-willy@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>