buffer: replace obvious uses of b_page with b_folio

These cases just check if it's NULL, or use b_page to get to the page's
address space.  They are assumptions that b_page never points to a tail
page.

Link: https://lkml.kernel.org/r/20221215214402.3522366-3-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>