fuse: refactor finished writeback stats updates into helper function

Move the logic for updating the bdi and page stats for a finished
writeback into a separate helper function, where it can be called from
both fuse_writepage_finish() and fuse_writepage_add() (in the case
where there is already an auxiliary write request for the page).

No functional changes added.

Suggested by: Jingbo Xu <jefflexu@linux.alibaba.com>

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