fuse: use the folio based vmstat helpers

fuse: use the folio based vmstat helpers

In order to make it easier to switch to folios in the fuse_args_pages
update the places where we update the vmstat counters for writeback to
use the folio related helpers.  On the inc side this is easy as we
already have the folio, on the dec side we have to page_folio() the
pages for now.

Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>