fuse: write back dirty pages before direct write in direct_io_relax mode

In direct_io_relax mode, there can be shared mmaped files and thus dirty
pages in its page cache.  Therefore those dirty pages should be written
back to backend before direct io to avoid data loss.

Signed-off-by: Hao Xu <howeyxu@tencent.com>
Reviewed-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>