cachefiles: narrow the scope of flushed requests when releasing fd

When an anonymous fd is released, only flush the requests
associated with it, rather than all of requests in xarray.

Fixes: 9032b6e8

 ("cachefiles: implement on-demand read")
Signed-off-by: Jia Zhu <zhujia.zj@bytedance.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://listman.redhat.com/archives/linux-cachefs/2022-June/006937.html