fuse: update stats for pages in dropped aux writeback list

In the case where the aux writeback list is dropped (e.g. the pages
have been truncated or the connection is broken), the stats for
its pages and backing device info need to be updated as well.

Fixes: e2653bd5 ("fuse: fix leaked aux requests")
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Cc: <stable@vger.kernel.org> # v5.1
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>