fuse: copy_file_range should truncate cache

After the copy operation completes the cache is not up-to-date.  Truncate
all pages in the interval that has successfully been copied.

Truncating completely copied dirty pages is okay, since the data has been
overwritten anyway.  Truncating partially copied dirty pages is not okay;
add a comment for now.

Fixes: 88bc7d50 ("fuse: add support for copy_file_range()")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>